Per Alessandro Castellani, Thunderbird is "bunch of code running on top of Firefox." The Thunderbird extension site opens up a new *browser* tab *within* Thunderbird itself. Following this train of thought, why can't I open links in new tabs within Thunderbird itself?

Because the more web pages you open in browser mode, the more likely you are to bump into issues that have been addressed by the Firefox team, but not the Thunderbird one.

Allesandro is correct in saying Thunderbird runs on the Mozilla platform. He is not correct in saying it is running on Firefox. They are not quite the same thing.

Just as chromium the open source browser project developed a rendering engine. Google chrome and Microsoft Edge, are both also using the chromium engine, but the three things are not quite the same thing. Just as Microsoft is using the chromium engine Chrome uses, Thunderbird uses the Mozilla engine (used to be referred to as Geko) that underlies Firefox.

But Thunderbird does have a lot of the necessary and it is getting a little more focus as Oauith2.0 requires a browser authentication process as it is a web protocol being shoehorned into email by web companies (google first then Microsoft and yahoo got on the bandwagon. but there were no navigation button until about version 78

Say I have an RSS feed link that links to another article. I click on that, and a whole new browser window opens up instead of staying within the Thunderbird application. Rather frustrating to see, considering how Thunderbird *is* capable of displaying web pages.

Perhaps, but once you click the link you are in browser territory, not longer in feed reader territory. Even as a feed reader displaying HTML feed pages, Thunderbird's rendering is quirky at best (and I use it, but I do not love it)

Supposedly, extensions like Link in Tab fix this. More power to you if you've gotten them to work, but it's useless for me. I've tried fiddling with about:config and the TB config editor, but neither one worked for me. Any other resources I've come across through search is either mildly outdated (i.e. 2019-2021) or horrifically outdated (i.e. 2008-2012).

I just installed https://addons.thunderbird.net/en-US/thunderbird/addon/link_in_tab/ the next link I clicked it opened in a tab. I have now disabled it as it is not behavouur I like, but given hopw easily it worked for me, the issue may well be in your computer, or perhaps is a feature of some security software. It works for me is all I can say here.

Is this feature that I'm looking for completely nonexistent within Thunderbird itself, or am I just doing it wrong this whole time and I need someone to point out the obvious solution? I am desperately hoping for the latter, but my intuition is telling me it's the former.

I really don't know, as I said the addon you mentioned just installed and worked for me on Windows 11. But I do not have any third party security software that might block an unknown browser from connecting to the web.

Thank you for the thorough response. A crying shame that I must flick between Firefox and Thunderbird, but a different conversation for a different day. I have a modified user profile on Firefox, so that might explain where things might be going awry. Nevertheless, don't have enough patience to figure out why LinkInTab isn't working for me when pivoting back to the RSS reader extension in my web browser will have to do anyway.