I can't believe i never thought of using command line switches for that kind of stuff, i always just have firefox set to forget all data on close.
Not exactly a bash alias, but might be useful too: Since I often get bug reports for specific versions of firefox, i've set up a small script for downloading and starting a given firefox version:
https://pastebin.com/t2PHxJqE
I have a similar command, except it keeps the existing addons from my main profile. Which is useful as I need a proxy for different environments for work.
Not GP, but you could just do the same except after making the temp dir: copy in `profile/addons.json` and `-r profile/extension*` from your main profile. (There might be other files needed to, not tested.)
Super useful for testing sites or add-ons without influencing (or being influenced by) by my normal browsing profile.