LibreWolf is also available as an AppImage. After reading this news today, I downloaded that AppImage, started it up and closed it down (just to create a profile folder), and replaced the default contents of the new LibreWolf profile folder with a copy of the contents of my existing Firefox profile folder. I then started LibreWolf up and went into Edit-->Settings and inspected all of those (they didn't carry over for some reason so I had to check and edit them manually). Other than that, it's working fine so far: my old tabs / extensions / passwords are all here, and I'm writing this comment from LibreWolf! (I've kept my Firefox profile folder for now just to be safe, but have otherwise uninstalled Firefox.)