I have a vault (CryFS, standard settings) where I store the profile folder of my Firefox. The profile folder is soft linked to /home/...user.../.mozilla/firefox/...profile... Sometimes Firefox freezes completely. It doesn't seem to depend on any specific action. It is also independent of the Firefox' version. The process of Firefox cannot be killed, it is necessary to reboot the computer. If the profile folder is on it's regulary place (/home/...user.../.mozilla/firefox/...profile...) then everything works fine. Kubuntu 18.04 plasmashell 5.12.6 Qt: 5.9.5 KDE Frameworks: 5.44.0 kf5-config: 1.0
When a crash has happened, there is often a file that cannot be deleted. It doesn't help to unmount and remount the vault. It is necessary to create a new vault and copy all files (except the broken one).
For testing purposes I moved the Firefox profile folder into a Veracrypt container. This works without any problems.
This is not something that we can fix - if a firefox crashed, it is a bug in firefox. Now, for this answer no to be completely useless to you :) you can create a firefox profile in a specific directory - you should not need to have any symbolic links at all. When creating a new profile, just click the 'Choose folder' and choose a folder inside of a mounted vault.