Install Firefox with Global Menu patches on Ubuntu derived Linux distributions. https://alexdelorenzo.dev/notes/firefox-global-menu

install_ff.sh

#!/usr/bin/env bash
# Copyright 2022 Alex DeLorenzo. Licensed under the AGPLv3.
#
# This script requires Bash, byobu, curl, and pup. You can find the latter here:
#   https://github.com/ericchiang/pup
# For more information, see https://alexdelorenzo.dev/notes/firefox-global-menu
#
# Usage:
#   $ ./install_ff.sh amd64
readonly ARCH="${1:-amd64}"
readonly URL="https://packages.ubuntu.com/bionic/$ARCH/firefox/download"
readonly DEB_PATH="div#content > ul > li > a attr{href}"

readonly debUrl="$(curl -s "$URL" | pup "$DEB_PATH")"
readonly debFile="$(basename "$debUrl")"

# download if file does not exist
test -e "$debFile" || {
  printf "Downloading %s\n" "$debUrl"
  curl -O "$debUrl"
}

# get current and downloaded firefox versions
currVer="$(apt show firefox | grep Version | col2 | cut -d- -f1)"
downloadVer="$(dpkg -I "$debFile" | grep Version | col2 | cut -d- -f1)"

printf "Installed: %s\n" "$currVer"
printf "Downloaded: %s\n" "$downloadVer"

# check if installed version is different from downloaded version
test "$currVer" != "$downloadVer" && {
  # install
  sudo apt install ./"$debFile"

  # hold version
  sudo apt-mark hold firefox
}

Some dependencies are needed.

sudo apt-get install byobu

Also install brew imo and do brew install pup to save the hassle of trying to get it up and running.

There is also an issue of the close, minimize and restore/maximize buttons disappearing with this fix as well.

Possible to work around this by customizing your toolbar and checkmarking show titlebar or maybe a more proper fix, but it involves editing the css of Firefox to bring back the normal buttons without the title bar.

https://www.reddit.com/r/kde/comments/nm1vub/comment/gzpj9x4/?utm_source=reddit&utm_medium=web2x&context=3

Thanks for the heads up about the byobu dependency, @rbreaves.

I haven't had problems with the sizes of the window buttons, but that might be because I'm using KWin and its default window decorations, and not Firefox's.

pup is packaged for Sid, as well