Needed help to finalize Firefox as default browser.

If the Make Default button Robert mentioned isn't there or doesn't stick, I would do it from the Windows side instead, since that is what finally worked for me. In Windows 10 I went to Settings, then Apps, then Default apps, and picked Firefox under Web browser. That setting is the one Windows actually honors, so it is worth checking even after clicking the button inside Firefox. If Firefox is already listed there, then it is already the default and the in-browser button simply has nothing left to change.