If all extensions are disabled and their Enable buttons are grayed out or inactive, Firefox might have restarted in its Safe Mode. If you need to preserve your previous session tabs and haven't already restore the session, you should go ahead and do that (from the History menu) before exiting Firefox and starting it up again normally.

If there is a mix of enabled and disabled extensions that can't be re-enabled, hmm, something might have gotten corrupted.

I don't think that Firefox is in safe mode, though I don't know how to determine that. The ePub extension is now enabled, however. Avast, Logitech and Skype are disabled with no buttons [Remove or Enable] shown. LastPass is disabled with only a Remove button. When I check for updates, no updates are found.

Could you try removing the files in which Firefox has stored information about your extensions. The steps are in cor-el's post here: https://support.mozilla.org/questions/1052141#answer-704047

I think Firefox will regenerate the files the next time you start it up again.

I used "Help [click ? in Firefox menu] > Troubleshooting Information > Profile Directory: Show Folder" to open the profile folder. I then found and deleted all of the extension related files that had been created earlier today. Then I restarted Firefox. Tabs opened for each of my extensions allowing me to enable all of them. Problem solved:) Thanks!!