Playing around with mousewheel.withcontrolkey.action, I set it to 1 -- which should be 'scroll up and down' -- TB indicated that this was a user override after I made this change, so I know it's not the default setting. However, when I tried to scroll up and down with that setting, it zoomed in and out, instead.

So I have the functionality I want, but there remains some issue somewhere!

Thanks, --J

If it worked properly in safe mode, and it's not a result of add-ons, consider the other implications of safe mode and try to eliminate the possible sources of the problem, e.g. hardware acceleration, broken layout etc.

http://kb.mozillazine.org/Safe_mode

I haven't been able to track anything else down. There has been no hardware acceleration on at any time, and I've reset the layout. My workaround stopped working, and I have made no progress at all.

I concur that there's something in the profile that is causing this, but I have nothing customized. Completely blowing away the profile appears to be the only solution, and one I'm not particularly happy about, as I am using a number of accounts.

Thanks, --J

You can create a new profile with Profile Manager, set up one account, and see if the mouse works correctly. If it doesn't, the fault probably lies outside of TB, perhaps with the mouse itself. If it works correctly, you can transfer the mail, calendars and contacts from the old profile to the new one.

Created a new profile, and all was working well. However, this morning, mousewheel+ctrl zooming is broken in the new profile.

I haven't changed any settings... I've just re-added my accounts.

Ctrl+mouse works as zoom for me here in MX Linux. Which DE are you using? I am on Mate.

If you are habitually zooming to make text legible, there are better ways of enlarging text by setting text/font sizes in the display settings.

It's really funny... sometimes it will work, and then it will stop. I'm on a 4K monitor, so I'm finding I have to do more zooming in and out, as differences are even greater. And I find I want to scan some email in small print, examine other email in larger print, etc.

I'm on stock Ubuntu 16.04.2 LTS. I have two machines running side-by-side -- one works reliably, the other one doesn't, with nearly identical configurations.

I'll just keep digging and see where I get.

Thanks! --J

The sporadic nature of this problem and the comparison of the two machines suggests a mouse hardware issue, which could possibly be related to a graphics driver if you have a high-end card driving the 4k monitor. Do the keyboard equivalents work, Ctrl++ and Ctrl+-? In any case, the advice given above to adjust your display fonts is recommended:

http://xenos-email-notes.simplesite.com/417754775

http://kb.mozillazine.org/Font_settings_in_Thunderbird

Ctrl-+ and Ctrl-- both work as expected.

Changing the font sizes, while making things bigger, doesn't solve the problem. I tried it anyway, but zooming is pretty important for the types of emails I seems to received (and my aging eyes).

No sense spending more time on this, I think I'll just live with it for a bit and see if I can adjust to the keyboard shortcuts instead.

Thanks, --J

tried changing the use hardware acceleration in options. It can be one of those setting that on works for one driver revision and off is required for the next. It also produces weird issues. Hence me mentioning it.

Well... nothing seemed to fix things except running in safe mode. Whenever I would create a new profile, the problem would not appear, but would return after a few sessions.

However, I changed to a new theme (monterail), the ctrl+mouse zooming now works as expected.

Obviously, there is something either in the default theme (as it would return in a new profile) or settings that I could not track down.

Thanks for the help, everyone! It's greatly appreciated. --J