its almost impossible to search google and get any useful result as it thinks i want to know about ssd writes or RAM usage. Duckduckgo wasn't much better. I am sure it was easier to find things before google search but I digress...
Very few people get this problem. main reason we haven't got an answer yet.
Now its likely the folder that is growing is in your user folder
Type
about: profiles into search bar, (there is no space between : and p but stupid forums replaces it with an emoji if i don't add the space, so remove it when you type it)
if you anything like mine, its either
Root directory -
C:\Users\Colif\AppData\Roaming\Mozilla\Firefox\Profiles\ta2ou1yb.default
Local directory -
C:\Users\Colif\AppData\Local\Mozilla\Firefox\Profiles\ta2ou1yb.default
The number at end will be different
from the about screen you can open the folder directly
Storage folder is in the roaming folder choice, this is where you need to look. I could guess its the temporary folder since it empties on exit. You might need to run windirstat as admin for it to show folder.
this might help -
https://www.nirsoft.net/utils/mozilla_cache_viewer.html
how many tabs do you normally have open? how many arre youtube? 10gb is an awful lot of data...