SUMMARY Each time I start Firefox I get the window asking to make Firefox my default browser. I did not change the default browser, nor I have installed other browsers. I found out I have a /usr/sbin/firefox that sets the GTK_USE_PORTAL variable. If I start firefox by using /usr/bin/firefox instead, I have no dialog and Firefox is happy. Probably related to bug 402206. I have the lastest Neon stable with all updates installed as of today.
This is an upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1516290
*** This bug has been marked as a duplicate of bug 425287 ***