How do I get my old firefox profile to run in snap environments of re-installations/other users on my machine?

My ambition is to reinstall my system and then, in the fresh installation, restore the home-folder of my current main-user from its backup created with borg. But before I boot the live USB stick, format the drive to commence this, I thought that I first wanted to make sure that I will be able to properly access my firefox profile as well as my mail profiles in thunderbird. In preparation I created a new user, let's call it user2 in possession of its' own /home/-folder and set up the file permissions between main-user and user2 such that I can create backups through the user2-account into that user's home-folder of files and folders owned by my actual main-user-account. (because they both are in group borg and chown -R 750 ... and chown -R 770 ... respectively) But NOW to solutions for my actual problem. My approach was to simply try to copy the firefox profile folder in /home/main-user/firefox/current/ to /home/user1/firefox/current/ with rsync and adjust the profile.ini file accordingly. This doesn't work with the sandboxing of the /snap-directory very well obviously ! Either snapd seems to prevent these sort of things as the folder is only a SquashFS mountpoint for stuff that is actually maintained in /var/lib/snapd/snaps/ Through issuing readlink /home/main-user/snap/firefox/current I found out that the firefox version in current points to the 3779-Version and thus probably either to /home/main-user/snap/firefox/3779 or to /var/lib/snapd/snaps/firefox_3779.snap (as mount source or something?) Could you please tell me how I properly get my current firefox profile to work in another snap sandbox / mounted package either after re-installation or simply because I want in another user's /home/userX/snap/ directory ? Big Thanks