This sounds like a Firefox UI issue, not a Startpage one. If you need help uninstalling our extension or anything Startpage related, get in touch at support@startpage.com. Hang in there.

And here's where to leave feedback for Firefox to add us as a default option (meaning an extension would not be required to use Startpage as the default search engine in Firefox).

https://connect.mozilla.org/t5/ideas/add-startpage-private-search-as-option/idi-p/72587