This happens if it's outdate or no longer supported version it's removed and you have to download more up to date Addon. Otherwise you need to contact Google support Addon and ask what is going on with their Addon when used in Firefox.

The "To Google Translate" extension is not made by Google though. https://addons.mozilla.org/firefox/addon/to-google-translate/

Hi Gene, normally the only time your Firefox would rid itself of an extension is if Firefox performed a Refresh, which gets rid of all your extensions.

In rare cases, an extension that is blocklisted and deemed to be dangerous might be removed, but since "To Google Translate" is a Recommended extension, that obviously is not the scenario here.

It's difficult to think of any other reason for an add-on to be removed (unless your Firefox is a managed installation and your IT department did it).

Thanks jscher2000 It's not a managed install, but I've taken a screenshot or the add-ons page so I'll know what's been taken out. It's a minor annoyance and reinstall is easy.

I'll leave this post open for a few days in case there's any more ideas.