A script to download and unzip geckodriver, and then add it to the user path.

download_geckodriver.ps1

$ProgressPreference='SilentlyContinue'

Write-Host "Finding OS Architecture: " -ForegroundColor DarkCyan -NoNewline
# Find Os Architecture
if((Get-ComputerInfo).OsArchitecture -eq "64-bit"){
    $arch = "*win64*"
    Write-Host "64-bit" -ForegroundColor Green
}else{
    $arch = "*win32*"
    Write-Host "32-bit" -ForegroundColor Green
}

Write-Host "Making Temp Directory: " -ForegroundColor DarkCyan -NoNewline
# Generate random 10 char string
$tmp = -join (((48..57) + (65..90) + (97..122)) * 120 | Get-Random -Count 10 | ForEach-Object {[char]$_})
# Make temp directory
$tmpDir = "$env:TEMP\tmp.$tmp"
mkdir $tmpDir *>$null
Write-Host "$tmpDir" -ForegroundColor Green

Write-Host "Downloading zip: " -ForegroundColor DarkCyan -NoNewline
# Download zip
$tmpLocation = "$tmpDir\geckodriver.zip"
$downloadUrl = (((Invoke-WebRequest -Uri "https://api.github.com/repos/mozilla/geckodriver/releases/latest" -UseBasicParsing).content | ConvertFrom-Json).assets | Where-Object name -Like $arch).browser_download_url
Invoke-WebRequest -Uri $downloadUrl -OutFile $tmpLocation -UseBasicParsing
Write-Host "Done!" -ForegroundColor Green

Write-Host "Extracting exe: " -ForegroundColor DarkCyan -NoNewline
# Unzip
Expand-Archive -LiteralPath $tmpLocation -DestinationPath $tmpDir
Write-Host "Done!" -ForegroundColor Green

Write-Host "Checking if gecodriver already exists: " -ForegroundColor DarkCyan -NoNewline
$installDir = "$env:APPDATA\geckodriver"
$shouldCopy = $true
if(Test-Path "$installDir\geckodriver.exe"){
    Write-Host "gecodriver already exists" -ForegroundColor DarkYellow
    Write-Host "Checking file hashes: " -ForegroundColor DarkCyan -NoNewline
    if((Get-FileHash "$installDir\geckodriver.exe").hash -eq (Get-FileHash "$tmpDir\geckodriver.exe").hash){
        Write-Host "Hashes match!" -ForegroundColor DarkYellow
        $shouldCopy = $false
    }else{
        Write-Host "Hashes doesn't match" -ForegroundColor Green
        Write-Host "Removing old exe: " -ForegroundColor DarkCyan -NoNewline
        Remove-Item -LiteralPath "$installDir\geckodriver.exe" -Force
        Write-Host "Done!" -ForegroundColor Green
    }
}else{
    Write-Host "gecodriver doesn't exist" -ForegroundColor Green
    Write-Host "Creating install directory in AppData: " -ForegroundColor DarkCyan -NoNewline
    if(!(Test-Path $installDir)){
        mkdir $installDir *>$null
    }
    Write-Host "Done!" -ForegroundColor Green
}

if($shouldCopy){
    # Copy exe from temp directory to install directory
    Write-Host "Copying exe: " -ForegroundColor DarkCyan -NoNewline
    Copy-Item -LiteralPath "$tmpDir\geckodriver.exe" -Destination "$installDir\geckodriver.exe"
    Write-Host "Done!" -ForegroundColor Green
}

Write-Host "Cleaning Up: " -ForegroundColor DarkCyan -NoNewline
# Clean Up
Remove-Item -LiteralPath $tmpDir -Force -Recurse
Write-Host "Done!" -ForegroundColor Green

Write-Host "Adding gecodriver to Path: " -ForegroundColor DarkCyan -NoNewline
# Adding gecodriver to Path
$userPath = [System.Environment]::GetEnvironmentVariable("Path", "User")
if($userPath -like "*;$installDir*"){
    Write-Host "gecodriver already in Path" -ForegroundColor DarkYellow
}else{
    [System.Environment]::SetEnvironmentVariable("Path", "$userPath;$installDir", "User")
    Write-Host "Done!" -ForegroundColor Green

    Write-Host "Updating session Path: " -ForegroundColor DarkCyan -NoNewline
    # Update session Path
    $env:Path = "$env:Path;$installDir"
    Write-Host "Done!" -ForegroundColor Green
}

download_geckodriver_clean.ps1

$ProgressPreference='SilentlyContinue'
if((Get-ComputerInfo).OsArchitecture -eq "64-bit"){
    $arch = "*win64*"
}else{
    $arch = "*win32*"
}
$tmp = -join (((48..57) + (65..90) + (97..122)) * 120 | Get-Random -Count 10 | ForEach-Object {[char]$_})
$tmpDir = "$env:TEMP\tmp.$tmp"
mkdir $tmpDir *>$null
$tmpLocation = "$tmpDir\geckodriver.zip"
$downloadUrl = (((Invoke-WebRequest -Uri "https://api.github.com/repos/mozilla/geckodriver/releases/latest" -UseBasicParsing).content | ConvertFrom-Json).assets | Where-Object name -Like $arch).browser_download_url
Invoke-WebRequest -Uri $downloadUrl -OutFile $tmpLocation -UseBasicParsing
Expand-Archive -LiteralPath $tmpLocation -DestinationPath $tmpDir
$installDir = "$env:APPDATA\geckodriver"
$shouldCopy = $true
if(Test-Path "$installDir\geckodriver.exe"){
    if((Get-FileHash "$installDir\geckodriver.exe").hash -eq (Get-FileHash "$tmpDir\geckodriver.exe").hash){
        $shouldCopy = $false
    }else{
        Remove-Item -LiteralPath "$installDir\geckodriver.exe" -Force
    }
}else{
    if(!(Test-Path $installDir)){
        mkdir $installDir *>$null
    }
}
if($shouldCopy){
    Copy-Item -LiteralPath "$tmpDir\geckodriver.exe" -Destination "$installDir\geckodriver.exe"
}
Remove-Item -LiteralPath $tmpDir -Force -Recurse
$userPath = [System.Environment]::GetEnvironmentVariable("Path", "User")
if($userPath -like "*;$installDir*"){
}else{
    [System.Environment]::SetEnvironmentVariable("Path", "$userPath;$installDir", "User")
    $env:Path = "$env:Path;$installDir"
}

Windows 7 SP1 x64
PS 5.1

PS C:\Users\PC> Set-ExecutionPolicy RemoteSigned

Execution Policy Change
The execution policy helps protect you from scripts that you do not trust. Changing the execution policy might expose
you to the security risks described in the about_Execution_Policies help topic at
http://go.microsoft.com/fwlink/?LinkID=135170. Do you want to change the execution policy?
[Y] Yes  [A] Yes to All  [N] No  [L] No to All  [S] Suspend  [?] Help (default is "N"): y
PS C:\Users\PC> C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da9
0393d-3bf37d72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1
Finding OS Architecture: Get-ComputerInfo : Unable to find an entry point named 'GetFirmwareType' in DLL 'kernel32.dll'.

At C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da90393d-3bf37d
72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1:5 char:5
+ if((Get-ComputerInfo).OsArchitecture -eq "64-bit"){
+     ~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Get-ComputerInfo], EntryPointNotFoundException
    + FullyQualifiedErrorId : System.EntryPointNotFoundException,Microsoft.PowerShell.Commands.GetComputerInfoCommand

Making Temp Directory: C:\Users\PC\AppData\Local\Temp\tmp.QZUXAK4dHR
Downloading zip: Invoke-WebRequest : The request was aborted: Could not create SSL/TLS secure channel.
At C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da90393d-3bf37d
72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1:24 char:19
+ ... oadUrl = (((Invoke-WebRequest -Uri "https://api.github.com/repos/mozi ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebExc
   eption
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Invoke-WebRequest : Cannot validate argument on parameter 'Uri'. The argument is null or empty. Provide an argument
that is not null or empty, and then try the command again.
At C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da90393d-3bf37d
72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1:25 char:24
+ Invoke-WebRequest -Uri $downloadUrl -OutFile $tmpLocation -UseBasicPa ...
+                        ~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Invoke-WebRequest], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Done!
Extracting exe: Expand-Archive : The path 'C:\Users\PC\AppData\Local\Temp\tmp.QZUXAK4dHR\geckodriver.zip' either does no
t exist or is
not a valid file system path.
At C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da90393d-3bf37d
72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1:30 char:1
+ Expand-Archive -LiteralPath $tmpLocation -DestinationPath $tmpDir
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (C:\Users\PC\App...geckodriver.zip:String) [Expand-Archive], InvalidOpe
   rationException
    + FullyQualifiedErrorId : ArchiveCmdletPathNotFound,Expand-Archive

Done!
Checking if gecodriver already exists: gecodriver doesn't exist
Creating install directory in AppData: Done!
Copying exe: Copy-Item : Cannot find path 'C:\Users\PC\AppData\Local\Temp\tmp.QZUXAK4dHR\geckodriver.exe' because it doe
s not exist.
At C:\81f3d610295c69c777b512e4da90393d-3bf37d72a678c5d3934355937e84e9928de4ad09\81f3d610295c69c777b512e4da90393d-3bf37d
72a678c5d3934355937e84e9928de4ad09\download_geckodriver.ps1:60 char:5
+     Copy-Item -LiteralPath "$tmpDir\geckodriver.exe" -Destination "$i ...
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (C:\Users\PC\App...geckodriver.exe:String) [Copy-Item], ItemNotFoundExce
   ption
    + FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.CopyItemCommand

Done!
Cleaning Up: Done!
Adding gecodriver to Path: gecodriver already in Path

@i486 I'm sorry this is script was written and tested on Windows 10.
I'm not sure if it works on Windows 7 and I don't have what it takes to test it.