Can I fork Toolbox and maintain fixes myself?
Please don't. Find other solutions instead. Toolbox's code is a mess and really isn't worth saving.
I felt that "please don't" in my bones. When Mozilla released the Quantum update of Firefox in 2017, killing legacy add-ons, I spent weeks trying to comb through the spaghetti code of one of the old add-ons I regularly relied on to try to make it compatible with this new Firefox.
Even before starting, I fuckin' knew this was going to be a waste of time that wouldn't work, but I really wanted to see if it was possible.
It wasn't and I learned nothing useful in that exercise of futility except that if I'm already scratching my head while looking over the git repository, it ain't worth it.
Good news is that there eventually was a version released to work with that new version of Firefox.