Après quelques temps, j'ai désinstallé. Non que cette extension posait problème mais elle était sans grande utilité.
En revanche je viens de tomber sur ça : https://wiki.archlin.../Profile_on_RAM
Je me ferais bien au moins la première partie pour essayer :
Relocate cache only to RAM
When a page is loaded, it can be cached so it doesn't need to be downloaded to be redisplayed. For e-mail and news, messages and attachments are cached as well. Firefox can be configured to use only RAM as cache storage. Configuration files, bookmarks, extensions etc. will be written to drive as usual. For this:
open about:config in the address bar
set browser.cache.disk.enable to "false" (double click the line)
verify that browser.cache.memory.enable is set to "true" (default value)
add the entry (right click->new->integer) browser.cache.memory.capacity and set it to the amount of KB you'd like to spare, or to -1 for automatic cache size selection. (Skipping this step has the same effect as setting the value to -1.)
Main disadvantages of this method are that the content of currently browsed webpages is lost if browser crashes or after a reboot, and that the settings need to be configured for each user individually.
A workaround for the first drawback is to use anything-sync-daemon or similar periodically-syncing script so that cache gets copied over to the drive on a regular basis.