Yes, you can restart specific snap by using snap restart nameOfTheSnap command.
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Thanks â back then I have given up on this and just tried using the apt-repository of firefox instead, but this was also annoying because even after pinning the apt version, after every automatic update ubuntu would enable the snap version which does not work, requiring a manual apt update to make it work again 
Anyways, because more snap-only programs have the same problem I wanted to re-try fixing this problem with snap itself. My home directory is really in /data (/data/johann) where /data is a different device than where /home is mounted. However /home/johann is still configured as my $HOME but /home/johann is just a symbolic link to /data/johann.
So I tried to solve the problem using: sudo snap set system homedirs=/data/ (tried both with and withou sudo), but after this, when I try to run e.g. firefox I get the error:
cannot perform operation: mount --rbind /data/ /tmp/snap.rootfs_fgX5Wd//data/: Permission denied
My snapd version is 2.67.1+24.04
Have you tried using a bind mount instead of a symlink?