A bug has affected add-ons in Firefox causing them to be disabled. The problem is being caused by security certificate expiring. This certificate is used to sign quite a few add-ons and since it has expired, the add-ons are being disabled. There are two fixes for the add-ons disabled in Firefox problem. A hotfix by Mozilla, and one that other Firefox users are suggesting. Both work, so take your pick.
EDIT: Update Firefox now. Mozilla has released an update that addresses this bug. You can skip the solutions given below and simply upgrade the browser.
Fix add-ons disabled in Firefox
Do not uninstall the add-ons or your browser. Doing so will not fix the problem and may cause you to lose data saved by the add-on. There is no fix available yet for Firefox Android from Mozilla though you can try the alternative fix for the mobile version of the browser.
The hotfix is being applied by Mozilla but you will need to make one small change to your Firefox settings. Open Firefox and click the hamburger button at the top right. Select Options from the context menu. Select Privacy & Security from the tabs on the left. Scroll down to the Firefox Data Collection and Use section.
Under this section, you will find an option called ‘Allow Firefox to run and install studies’. According to Firefox, it can take up to 6 ours for the fix to be applied.
To check if it has been applied, type about:studies in the URL bar and tap enter. Look for a study named hotfix-reset-xpi-verification-timestampe-1548973 . If it is listed there, the fix is working.
Disable signature enforcement
This alternative fix is basically to disable signature enforcement. Open Firefox and paste the following in the URL bar.
Accept the warning and then use the search bar to look for the following preference;
Double-click to set its value to False. Restart Firefox to be safe.
This fix comes with a risk. You should change it back the second Firefox resolves the problem and in the meantime, you should not install any new add-ons. Signature enforcement is important to make sure the add-ons you are installing are from a trusted source and have been verified by Firefox.
Temporarily load add-ons
This fix is a bit complicated to apply. Click the hamburger icon and select Help from the menu. Go to Troubleshooting information. Scroll down to the Profile Folder field and copy its path to your clipboard.
Next, enter the following in the URL bar;
Click Load Temporary Add-on button and navigate to your profile folder. Inside the folder, you will see an Extensions folder. Select the XPI files in this folder. You will have to select and load them one by one. The XPI files are add-ons.