It sounds as if your extension handling in the profile is damaged. You could try a new profile and then add to it everything but extensions, and install those manually one by one. Or see this page:

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.json, extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search bar to check if there is a compatibility update available.

If this hasn't helped then also delete possible addons.json and addons.sqlite files.

I deleted everything and redid the installation. When I installed Firefoix, it came up without delay. After installing just one add-on, I used BBCodeCtra 0.32, Firefox took about 5 minutes to load.

When I did this, I had Windows Task Manager opened and I observed that my Anti-Virus, Symantec Endpoint Protection (edpa.exe), was using up about 25% of the CPU. Firefox did not come up until edpa.exe when to 0%. So it appears that the add-on code is being scanned for viruses that is causing Firefox to load slowly and the more add-ons used, the longer it will take to perform the scan. Unfortunately, there is nothing I can do about Symantec Endpoint Protection. Removing or disabling it is not an option for me.

Then you will have to contact Norton for support or accept the delay.

There shouldn't be any need to scan extensions that you installed yourself in Firefox.