Cannot install search engines with "Unity Desktop Inegration" enabled

[Ben Roberts]

I am setting up a new profile on a new Ubuntu installation. After enabling Sync in Firefox (so it's possible something else in my configuration is contributing to this) I was unable to install search engine add-ons like https://addons.mozilla.org/en-US/firefox/addon/startpage-https-privacy-search/

When I attempted to do so, I received a pop-up dialog that stated "Sorry, you need a Mozilla-based browser (such as Firefox) to install a search plugin."

Using Firefox in Safe Mode caused the problem to go away, so I tried disabling my add-ons one by one. The one that was causing the issue was "Unity Desktop Integration" since when I disabled it, I was able to install a search engine.