After I upgraded my system to Kubuntu 24.04 thunderbird started eating CPU (150+% CPU usage according to htop). Because I couldn't tell if the culprit was the new KDE or the old profile, I tried to create a new profile, blow away the KDE config, etc. I also tried downloading the latest version of thunderbird from the website. Nothing helped. I was able to replicate this problem using the in-memory, fresh KDE. To replicate: Create Kubuntu 24.04 install USB. Boot from this USB. When asked to install or try, select "Try Kubuntu". Start Thunderbird. When prompted to create an account, select cancel. Go into settings, to the bottom, and select Maildir for file storage. Quit thunderbird, and start it again. Create account (select configure manually, enter the information for imap and smtp host, STARTTLS, detect authentication. My username does not contain an @, if that makes a difference). Select "Re-test" Try to read mail. You will quickly notice that Thunderbird is consuming > 100% CPU, and UI is non-responsive. Restarting thunderbird does not help. netstat -tupane shows one or two connections to my imap server port 143, but wireshark doesn't show any traffic on port 143 Not that I think it matters (since I can't get Thunderbird to start seriously downloading from the server) but I have a big ( >100G) Maildir behind my courier server. Note that I see this problem both with the version on Kubuntu 24.04 install USB, and the latest version downloaded from the thunderbird website.