Needed help to finalize Firefox as default browser.

In Firefox, open Settings, go to General, and under Startup click Make Default if that button is shown. On Windows, also open system Settings, go to Apps, then Default apps, select Firefox, and set it for web links and common web file types. After that, restart Firefox and click a web link from another app to confirm it opens in Firefox. If another browser keeps taking over, repeat the system Default apps step, since that setting can override the choice inside Firefox.

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.