Found this online...

xdg-settings get default-web-browser

$ xdg-settings set default-web-browser firefox-esr.desktop

Found this online... xdg-settings get default-web-browser $ xdg-settings set default-web-browser firefox-esr.desktop

Thanks for the response. My understanding of xdg-settings is that it's basically a command line version of the equivalent GUI setting tools, so setting the default browser through that would yield the same result as doing it through the GUI tools. I'm able to set the default browser, and the system shows that it's set as I specified. But when Firefox is spawned by an application, like Thunderbird, Firefox and Thunderbird think there is no default browser specified, even though the system says it is.

That cmd was just to verify that the OS is seeing Firefox as default.

Also try downloading Firefox from Mozilla, run it from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

I just changed my default from my distro version to Mozilla version and it works. see screenshot Switching it back to distro version, opens the distro version of Firefox.

Why the old version of Thunderbird?

Why the old version of Thunderbird?

I generally like to stick with the packages tested and maintained by the distro unless a newer one has features I really need or fixes a bug that's a problem for me. Also, when there's major changes in the code base, it can be a problem for extensions I rely on. I actually went through that with a certain browser.  :-)

That cmd was just to verify that the OS is seeing Firefox as default.

I was checking that in the GUI utility. The OS sees Firefox as the default, even when Firefox is complaining that it isn't. That's what suggested that Firefox might be spawned in a shell that isolates it from the system settings

I just changed my default from my distro version to Mozilla version and it works. see screenshot Switching it back to distro version, opens the distro version of Firefox.

When you say "it works", do you mean the newer version of Firefox is functional, you're able to switch between versions, or the new version solves the problem in the question?

The more I look at the problem, the more it isn't clear where the problem lies (Firefox acting senile, Thunderbird having a bug in the way it spawns Firefox, or the OS having a bug). It will take some troubleshooting to eliminate some of the possibilities, or just to discover a version that works. I also want to check the various support forums to see whether other people are reporting the same issue.

I was hoping it might be recognized here as a problem other people were reporting, or something Mozilla was aware of. That could lead to a ready fix, or advice to just wait for the planned fix.

I discovered this issue because it was causing a weird problem in Thunderbird. When Thunderbird saw the same indication of no default browser that Firefox saw, it treated links in emails as "feeds", stuffing them into a special mailbox folder in the profile, and then reporting that you were already subscribed to that feed. It took some troubleshooting to identify the problem.

I created a question on the Thunderbird support site to figure out a solution to the Thunderbird manifestation. Someone there suggested explicitly specifying Firefox as the browser in the Thunderbird settings, so Thunderbird wasn't relying on the default browser setting. That worked, but Firefox still complained that it wanted to be set as the default browser. Since the error was harmless at that point, I just told Firefox to stop asking.

So the workarounds hide the symptoms, but don't solve the underlying problem. It takes the pressure off the urgency for me to troubleshoot it.

Yes, I can switch the setting between the distro and Mozilla Firefox. I don't use Thunderbird, but let me install it. My bad, I forget that I'm on a rolling release. How about downloading a newer version? Run it from the folder and test.

https://www.thunderbird.net/en-US/

How about downloading a newer version? Run it from the folder and test.

This issue is eating my lunch. The problem is actually on my wife's computer, which is new. It took a lot of time to get her set up initially, then she discovered the Thunderbird problem, and it took another big chunk of time to reach this point.

I posted this question hoping this was a known issue and there was a ready-to-go solution. Now that we have a workaround, my wife has other priorities for me that previously got delayed.  :-)

I was planning to run a battery of tests, including trying a different version of Thunderbird, a different browser, spawning Firefox from a different application, etc. If the problem lies with Thunderbird, I'd prefer to test a viable replacement rather than do an academic diagnostic, so I need to check on reported issues and add-on compatibility. The testing will require putting everything back to its previous state, running the tests, possibly restoring the workaround, and then testing everything again to ensure it's all working. So I need a substantial block of time that coincides with when I can have access to that computer (and my wife considers this to have a lower priority than the "important tasks").

I also wanted to assess whether other people are experiencing this problem. If so, I don't mind running the diagnostics to find a real solution. But if it looks like it's some oddball behavior limited to that one machine, we already have a workaround, so there are better uses for the time that would take.

In any event, it would be a few days before I could do this. Any results I find, I'll post back here.