From the firefox search engine page (https://addons.mozilla.org/en-US/firefox/search/?atype=4), I get the following error alert message when I click "+ Add to firefox":
Sorry, you need a Mozilla-based browser (such as Firefox) to install a search plugin.
I've found 2 temporary solutions so far that work fine, but they're workarounds and not exactly an answer to as why this is happening:
Temp solution #1 is to right-click on the "+ Add to firefox" link, and "save link as" the xml file to /usr/lib/firefox-addons/searchplugins and restart firefox.
Temp solution #2 use firefox in safe-mode by running firefox -safe-mode in a terminal.
