'''Environment:''' * Ubuntu 22.04 * Thunderbird 128.12.0esr '''Background:''' Yesterday (7/25/2025) Ubuntu ran an auto-update and appears to have updated tbird. At the time this did not affect tbird as it was up and running with the old version of code. Today (7/26/2025), upon starting tbird and clicking on the ''Calendar'' tab tbird goes into 100% cpu utilization. This issue appears to be related to changes in how recurring events are now listed under ''Calendar'' as individual events. For events with no ending date this new 'feature' now generates entries for the next ''100 years'' (eg, 26 July 2025 - 26 July '''2125'''). Steps I've performed in an attempt to address the 100% cpu utilization issue: # shutdown tbird, renamed ''calendar-data/local.sqlite'' (effectively erase all calendar events), start tbird, click on ''Calendar'' tab ... little/no cpu utilization; great, cpu utilization is back to normal but my calendar is gone # shutdown tbird, put ''calendar-data/local/sqlite'' back in place, started tbird, do '''''NOT''''' click on ''Calendar'' tab, [menu] ''Events and Tasks'' ->'' Export ''-> saved all calendar events to an external'' *.ics'' file; shutdown tbird, renamed ''calendar-data/local.sqlite'' (effectively erase all calendar events), start tbird, [menu] ''events and Tasks'' -> "Import" -> import all''''' (202) calendar events''''' from the ''*.ics'' file; tbird goes into 100% cpu utilization for about 5 minutes and then becomes unresponsive # repeat of previous bullet up until the ''Import'' step; at this point I imported a single recurring event (happens once a month with no end date); tbird successfully imported the '''''1 recurring event''''' and upon clicking on the ''Calendar'' tab ... I'm presented with '''''1200 individual event entries''''' -> once a month up through July '''2125''' # upon creating '''''1 event '''''that repeats on a daily basis, tbird goes into 100% cpu utilization for a few seconds while it generates more than '''''36,500 individual events''''' (up through July '''2125''') under the ''Calendar'' tab At this point it appears (to me) that tbird's 100% cpu utilization and unusable UI is related to tbird having a hard time managing my 202 calendar events (most of which are recurring events - mostly monthly but there are several daily and weekly events, too). This latest update of tbird is unusable! Please rollback this behavior of expanding recurring events into individual events, or at a minimum give us a setting that keeps tbird from expanding recurring events into individual entries under the ''Calendar'' tab. In the meantime, since tbird is running in a virtual machine, I'm rolling back to a previous snapshot and disabling Ubuntu's automatic updates of tbird software.