Open another browser. Have it declared as default. But do no check the option to check again. Close that browser. Now do the same thing in Firefox.

Thanks for that. However, it didn't work. Here's what I did to make Firefox open normally, but still displays the statement in "Options > General > Startup: Firefox is not your default browser" ... 1 I uninstalled Firefox completely, using Revo Uninstaller, and manually deleting all references of Firefox in the Registry. 2 Ran Chrome to check if it was now the default browser. 3 Re-installed Firefox from a new clean download. It asked the "default" question. 4 I unchecked "Always perform this check..." and clicked "Use Firefox as my default browser". 5 A box opened asking "How do you want to open this type of link (http)?" I chose "Keep using Firefox". Surprising, seeing as I only opened Firefox from Start and not through a link. 6 Once opened, I chose "Options" and noticed that the first item on the General tab: "Startup - Always check if Firefox is your default browser." was checked once again, and under it was the statement: "Firefox is not your default browser". 7 Without clicking "Make default" I closed Firefox and tried a web-link from Thunderbird. Firefox opened, so it was the default ! 8 In "Options > General" the check box "Startup: Always check if Firfox is your default browser" was not now checked, but the statement below still said "Firefox is not your default browser". It's clear something is still not quite right, but I can live with it now. I've noticed little problems with other programs, too, so perhaps it's windows 8.1 being awkward.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Thanks for that. I worked through some of the issues mentioned and now - lo and behold - Firefox now says it _is_ the default browser.

Glad to help.