I think it's a very good thing that Thunderbird got a major redesign of the UI. I would wish for a way to have native window decorations, however:
I'm on KDE and since the default KDE title bar is not there, I cannot open the window menu, I cannot close the window on double click and I'm missing all other customized settings, i.e.: Most buttons are missing (pin to all desktops, roll-up window, keep on front/back), the title bar does not display the window title and the height of the title bar is wrong.
A right click on the title bar should open a KDE menu -- as it does on all other applications -- but it opens a Thunderbird menu.
Thunderbird is in fact the only application on my system that does not have native KDE window decorations and does not respect my settings.
Am I the only one with this problem?
Unfortunately, it is not working very well in KDE. Hence, when using native window, then the search bar and the right icon bar are considered by kwin as “holes” in the window (i.e. when I click there, it clicks the window underneath TB!) It is very annoying.
I have found Firefox has certain quirks also. Not the "shaking issue" you experienced, but is always just slow to load anything, and YouTube videos take forever to start playing compared to browsers such as Chrome or Edge. The only reason I have stuck with Firefox is because it's the only browser where video does not "jitter" (appears to drop frames) especially in YT. Chrome does this if I maximize to full screen or pop-out a "pic in pic" window and resize it to more than half monitor screen size. For all its bells and whistles Firefox is not as great as many people make out. I am quite disappointed with it actually and if I find a solution to this "video jitters" I will swap back to Chrome
I mean for many people its okay to have these decorations (In TB and FF they are choosable to either native or Cliend side). But a big usability issue is that the close button is not in the exact corner. so I move my mouse there, but cant close it, I need to move it back etc.
This is completely bad UI, as the hitboxes could just be expanded. I have no idea why anyone would design something like this, except if the Desktop is not meant for use with a Mouse, which I dont really think it is.
So if it is possible, expanding the hitbox to the very corner would solve the problem for me already. It would look the sime, but actually work.