You should not need to reinstall anything after an update. Firefox does check extensions to see if they are compatible with the new version and reports any problems to you, sometimes with the option to update, and other times just disabling incompatible extensions. But they should not be removed.

Is it possible that the Refresh feature is running during your update? This could happen if Firefox detected that it was starting slowly and presenting an option to restore some default settings to speed things up. If you click that button, Firefox moves your settings folder to the desktop inside a folder named Old Firefox Data and there go your extensions, custom settings, etc. Do you see any indication of that?

Are the extensions really gone and show no longer in "Firefox/Tools > Add-ons > Extensions" or are they still there, but merely disabled?