Try to uninstall firefox, clean with a reg-cleaner. Restart. Use a malware cleaner, a browser cleaner and a Virus-Scanner. Install the firefox again, install also all extensions. Then you make a backup of your profile folder which have the extension in it.

When you should have this problem again, you have all ready. But it is better to clean your pc, first.

Which OS, do you have ?

Thanks. Actually, the problem doesn't always happen every time I restart, but sometimes it happens when I have more than 200 tabs open. Maybe I can try what you suggested and try cleaning my PC if it happens again. But usually, I can fix this problem by deleting extensions.json, extensions.ini, extensions.sqlite and sometimes addons.json and prefs.js as well. Thanks for your help. The OS I'm using is Windows 10.

Are you possibly using external cleanup software to clear data in the profile folder?

External software that messes with files in the profile folder is probably the main cause of files getting corrupted.

Hey, Sadolino. I had to fix the add-ons problem twice today. By the way, the operating system I am using is Windows 10 Home 64-Bit. And what external software do you think might be causing this to happen, cor-el?

Also, sometimes the add-ons stop working after I go onto multiple tabs. Do you have any ideas which registry cleaners I should use that are legitimate and aren't adware or malware?

Hello, I've found CCleaner as my legitimate registry cleaner, but do you know of any legitimate browser cleaners? Cleaning my registry didn't solve the add-on problem, but I didn't try uninstalling and re-installing Firefox.

Infernoflower1 said

I've found CCleaner as my legitimate registry cleaner

Just go into its options and have it leave Firefox alone.

Profile data can be removed or not recorded in several ways;

• 1) Running Firefox in Private Mode (nothing is recorded)
• 2) Privacy settings; (In the address bar, type about:preferences#privacy<enter>)
• 3) Many disk cleaners and anti-virus type programs can remove data from browsers. Open their settings and have them leave the browsers alone.
• 4) Mal-ware.