I am guilty beeing a tab messy and was searching for a while for a good solution to move my open tabs to OneTab. Especially on mobile the tab situation was extremely bad. When I realized that I had over 3000 (!!) tabs on my iPhone opened I acknowledge that I got to do something.
First I tried to just open the tabs in Firefox. When you rightclick a device on the "Synced Tabs" sidebar there is an "Open all tabs" entry.
For 3000 tabs already my computer was already overloaded when opening them… Closing every other open program and removing every browser extension other then blockers also didn't do the trick.
After some time the tabs seemed to be in an open state, but Firefox was not really responding. I tried closing the tabs via OneTab and also to save them all as bookmark via Bookmarks -> Bookmark all Tabs
but that also didn't work.