NOTE: Readme is pretty rough. I probably missed something important. If there's any public interest in the project, I'll polish things up.
Session Manager for classic Firefox hasn't been properly replicated, 'Tab Session Manager' is close but has some serious issues - likely because the same capability simply cannot be done via WebExtensions.
So out of sheer frustration with Waterfox + Session Manager suffering from bitrot and performance issues, I decided to finally migrate to Firefox and just make myself the one thing that was missing - and so SessionLauncher was born.
Leveraging the awesome power of Windows Batch and Internet Explorer HTA's (cough), I have made this Windows-only (sad-face) launcher that, by hooking the Firefox (ESR recommended) config.js system, provides a GUI to launch a specific session by swapping files around early in the Firefox profile load (before the session is even read, thankfully). The end result is a single profile in Firefox still - sharing all data - except for the "session" (current tabs) which are provided by the launcher.
In case it wasn't obvious - there is no need to ever manually save the session. Just ensure "Load last tabs" option is set in Firefox, and the Session Launcher will take care of the rest.
NB: Firefox ESR is recommended rather than mainline releases as I have no idea if/when Mozilla will deprecate any of the stuff that SessionLauncher requires. Note that it does NOT require sandboxing to be disabled - it doesn't actually touch config at all, just leverages the same "hook" - so I don't believe there are any security concerns regarding SessionLauncher.
IMPORTANT: Requires user symlink permission, or running as administrator. Windows 10 Pro only AFAIK - done in secpol.msc
To install, just download this repo, copy the contents of copy-to-Firefox-install-dir to your Firefox install directory, and run Firefox! A _Default profile will be created on first launch - this cannot be renamed or deleted and always appears at the top as a fallback. (Note that you can always edit the scripts yourself if you really want to change the _Default name).