This looks like a problem with the disk cache. You can try to delete the cache2 folder in the secondary profile location in "AppData\Local". *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Firefox uses two locations in the hidden "AppData" location for the Firefox profile folder. One location in "AppData\Roaming" for personal data like bookmarks and logins and another location in "AppData\Local" for temporary files like the disk cache. Secondary location used for the disk cache and other temporary files (Local Directory on about:profiles). *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>

Does it still happen with BitDefender disabled?