It would be helpful if you could provide specific info on what settings you have in Thunderbird:

• Menu app icon > Settings > General
• scroll down to 'Files & Attachments' section

Do you see any mention of http and https in the 'Content Type' column ? If yes: Either delete the setting or choose specific browser. then restart Thunderbird.

Please report back on what you see in the 'Files & Attachments' section regarding http or https.

Need to know what settings you currently have got set up.

• Menu icon > Setting > General
• scroll to bottom and click on 'Config Editor'

Look for the following preferences

• network.protocol-handler.warn-external-default
• network.protocol-handler.warn-external.http
• network.protocol-handler.warn-external.https
• network.protocol-handler.warn-external.ftp

All the above preferences need to have a 'True' setting.

Please confirm you have set up all of the above using a 'True' setting.

If you installed Chromium browser recently - (it was installed after Firefox) - then Chromium always has a habit of resetting itself as the default browser.

• Check 22.04 System Settings > Apps > Default

What does it say?

re :KDE Settings indicate that Firefox is my default browser

But have you opened Firefox and set it up as the default within firefox ?

Open Firefox Menu app icon > Settings In the General section Does it say Firefox is currently the default browser? If no then set as default.

Thanks for all of your suggestions. 1. There were 3 actions listed in the 'Files & Attachments' section. Two said "use default browser (chromium)" and one said "always ask". I tried to change the browser to Firefox, but I couldn't find the appropriate file, perhaps because Firefox is installed in Flatpak. I therefore deleted all 3 of the actions, but the result is that hyperlinks in mail messages cannot be activated by xlixking on them. Selecting "open link in browser" from the right-click context menu opens them in chromium, which leaves me with the workaround I was using before, selecting "copy link location" and pasting the URL in Firefox. 2. I had already set the preferences you listed to true, based on the suggestion on one of the pages I referenced in my original post. 3. Firefox is set as default both in Firefox itself and in KDE settings. I never installed chromium, at least not knowingly. Presumably it was already baked into Ubuntu Studio. Any other suggestions would be welcome! Thanks for your support!

P.S. The problem appears to be peculiar to Thunderbird. If I click on a link in a PDF (in Okular) the link opens in Firefox, as configured and as desired.

1. There were 3 actions listed in the 'Files & Attachments' section. Two said "use default browser (chromium)" and one said "always ask".

That explains why chromium was opening. The settings were set to open it.

2. I had already set the preferences you listed to true,

As you say " I therefore deleted all 3 of the actions" in Files & Attachments, those other config editor preferences set to true should mean when you click on a link, you should get asked what browser to use, so you can select Firefox. Does this occur or not?

I tried to change the browser to Firefox, but I couldn't find the appropriate file, perhaps because Firefox is installed in Flatpak.

I did locate this link which mentions how to find the firefox executable - it might help: https://askubuntu.com/questions/1417313/can-not-find-executable-path-of-flatpak-apps

should mean when you click on a link, you should get asked what browser to use, so you can select Firefox. Does this occur or not?

No. I also misled you. Firefox is installed as a snap package, a practice which has been standard in Ubuntu for a few years now. I did a web search and a local search for a file location and turned up the directory /var/snap/firefox but failed to find anything that looked like an executable. IAC, it seems like a bug in Thunderbird that it fails to respect both the system settings and those in Firefox. Should I report it as such? If so, where? Thanks for your help!

I did find this link which has some comments that managed to solve the issue: https://www.reddit.com/r/kde/comments/mmbjmj/cant_set_thunderbird_or_firefox_snaps_as_default/

bug reporting: https://bugzilla.mozilla.org/enter_bug.cgi

You could install Firefox from the Mozilla website. https://www.mozilla.org

Link on installing Firefox from mozilla official download: https://support.mozilla.org/en-US/kb/install-firefox-linux

A good link: https://www.omgubuntu.co.uk/2022/04/how-to-install-firefox-deb-apt-ubuntu-22-04

Many thanks for the additional tips. I'm not happy with the idea of modifying system variables like "$XDG_DATA_HOME" to make this work as it should, much less going through all of the other steps that RedBearAK describes. The reddit thread did help me to find what appears to be the firefox snap executable:

/var/lib/snapd/desktop/applications/firefox_firefox.desktop

so I tried making this the target of Thunderbird's "Files & Attachments" action for https, which Thunderbird had recreated pointing to Chromium. This was a challenge, because the "Select Helper Application" dialog insists that you navigate the entire directory tree to find an application instead of allowing you to paste a directory path somewhere. The result was no action at all when clicking on a link in an e-mail. Since I had placed a link to Firefox on my desktop and the "Select Helper Application" dialog helpfully offers "Desktop" in its left-hand common locations" pane, I also tried selecting Firefox there, but the results were the same. If I change the action to "Always ask", the results are still the same. Clicking on an HTTPS link in an e-mail does *not* produce a prompt asking what application to use. Other than this integration, I haven't experienced any of the complaints i have read about Firefox as a snap. In particular i find that it loads quickly enough, even though I am running Ubuntu Studio 22.04 LTS on an antique desktop PC and my favorite third-party add-ons, uMatrix and OneTab are working fine. The idea of using Firefox as a snap seems good to me. I like the automatic updates and the idea of running it in a fenced-off environment. Nonetheless, it appears that I will have to uninstall the snap version and install Firefox natively (as suggested in your last link) in order to get a maintainable system that works as intended. I have posted a bug and will first await a response to that before proceeding.

As a last resort, I have simply uninstalled chromium. This allowed me to find "Use Firefox Web Browser (default)" in the actions list for http and https. Selecting these achieves the desired result, but the setting does not persist for https. The next time I open Thunderbird, the action entry for https is empty.

P.S. Even though the action for https remains empty, clicking on an https link in an e-mail *does* now in fact open it in Firefox.

Given that the whole idea is that Thunderbird dumps the link to the operating system to do it's thing with url's (whatever that is). The issue you are chasing is one that is entirely in Linux or more specifically your distribution.

Windows users have no action for links in helper applications unless they have been playing around with hidden preferences. Generally speaking Linux GUI users should not either.

Are you implying that Ubuntu Studio added the actions in Thunderbird's settings? I have a hard time believing that. If the "whole idea" is, as you say, to leave things up to the OS, why does Thunderbird under Ubuntu (at least as of 102.8.0) include these settings which were overriding my system preferences? Any "playing around with hidden preferences" I have done was an attempt to remedy the problem and was recommended by other posters.