Thunderbird is on 128 now, it would be unusual for a Linux distribution not to be offering the most recent version. I suggest you try it as a number of performance changes are included.

Secondly indexing is something of a known performance hog and it is generally suggested folk disable the global indexer when trying to work out these sorts of issues. See https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

I do not see changing to maildir as a solution. However see https://support.mozilla.org/en-US/kb/maildir-thunderbird

Thanks for responding. After restarting the PC, Thunderbird seems to be back to normal. I am running Ubuntu Studio 22.04 and don't find Thunderbird under "Installed" in KDE Discover, nor do I find any "update" options in Thunderbird's "Settings". Something seems to be hogging my memory, so I am having a hard time researching the problem, as things are crashing left and right. I need to migrate to Ubuntu Studio 24.04, so I guess I'd better do it sooner than later.

P.S. According to https://www.betterbird.eu/ the Betterbird homepage: >> Note that Thunderbird 128 and hence Betterbird 128 is shipping with a broken backend, causing IMAP folder corruption under some circumstances. Currently, we do not recommend version 128 for productive use. Show more

History: In December 2023 the Thunderbird team greatly changed the internals of mbox file handling, introducing a number of regressions. Soon after, the first bug was filed, reporting IMAP folder corruption. Currently, this bug is tracking the issue.

Symptoms: IMAP folder corruption is easy to notice. The user clicks on a message, and the message isn't correctly display. Usually some raw message content is displayed, often a MIME boundary starting with --. Some users report that a different mail from the one that was clicked on in the message list is displayed. Note that since the Thunderbird folks forced version 128 onto unsuspecting users via an unnecessary update, they are getting a high volume of reports that people's e-mail can't be displayed any more.

Remedy: Repairing the affected IMAP folder (Right-click, Properties, Repair Folder) usually fixes the issue. That's fine for users with little data, it causes problems for users with large folders (more than 1 GB) since the repair downloads all messages again. Some Thunderbird users report that they need to repair they IMAP mailboxes multiple times per week.

Outlook: We were estimating that the problem would be fixed in version 128.3.0 due in October 2024, but the Thunderbird team have done precious little to address the issue. It is possible that 128.3.0 will still be defective. As a recap, Thunderbird 102 initially shipped with a broken backend, including causing data loss; Thunderbird 115 "Supernova-kaboom" shipped with a broken frontend; now in Thunderbird 128 we're back to a broken backend. Breaking news: Mozilla will support the 115 series until 115.21 in March 2025. Maybe Thunderbird 128 will be stable by then. << Perhaps this is why my Linux distro hasn't yet updated Thunderbird to version 128.