firefox --Profile /path/to/separate/folder
Firefox
A place to discuss the news and latest developments on the open-source browser Firefox
This is the answer. I use firefox profiles a lot. They do not only give you the separation of the profiles, but also makes it easier to manage. Want a profile for work? Make one, sign up for firefox sync with your work e-mail, now log in also on your work phone, and your profile is there too. Do the same thing for your personal profile/e-mail/phone.
I think it should also be possible to use named profiles
Would FF Profiles work? https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
This doesn't answer your question, since it already seems answered, but an additional step that may be helpful is to use a completely separate firefox derivative browser for one of those tasks.
I personally use Firefox for most of my daily browsing, but I have LibreWolf installed with separate configuration options (different proxy, etc). I could do the same with multiple Firefox profiles, but I prefer the separate icon as a reminder that my workflow is different in this particular browser.
I thought about this solution, as it is the "cleanest", however I need on total 4 firefox derivatives. Unfortunately, when looking deeply into the options, i haven't found 4 that are similarly trustworthy, well maintained etc. Also i have my firefox config fully figured out, it works and is as private as i want them, without some maintainer forcing their opinion on my use cases. Plain firefox is the easiest to configure, as it's like a blank start. However i might be wrong here and am open to suggestions :D
That's prolly the dirtiest solution