Post a crash ID from Help/More Troubleshooting, Crash Reports. An ID is a string starting with bp-...

In the meantime, try running TB in safe/troubleshoot mode (hold Option when launching TB).

I did as you suggested. The result was unsuccessful. The chronology follows. -I restarted Thunderbird in the safe mode as was recommended. -I opened a new email. -I inserted a JPG photo file in line, successfully. -I selected Format> Page colors and background. -I Selected Use custom colors and background. -I selected use custom colors. -I selected the background box. -I selected the choose a color box. -I then chose a color from the offered choose a color window. -I then clicked on the OK button in the other window. -At this point, the application locks up. No cash ID is provided. No background color was applied. -The Page Colors window was frozen behind the email window and was greyed out. It could not be accessed or activated. -It was necessary to force quit the application. It would not otherwise close. -The original email I was creating was lost. -The exact same result occurs each time the same process is used whether in the safe mode or regular mode. I should also note that it is not possible to enter a second email address into the Address Book.

So, does it crash or just hang? A hang or 'not responding' won't generate a crash ID. No Crash reports?

To show that this is a bug and not a profile flaw, create a new profile from Help/More Troubleshooting, about:profiles, add an account, and see if the issue replicates.

I guess that it is what you call a "hang". Everything stops and no changes can be made. No, there is no crash report. Just so you know, this app was downloaded directly from your web site and then I set it up. I placed the previously updated Thunderbird app into the trash and cleared ALL associated Thunderbird files before reloading the new version of Thunderbird. I am sorry, I am nor inclined to mess with profiles since there is only one new profile for this app. Why don't you simply replicate the steps i've outlined on a Mac as I did and you will also see the same result. It happened with the old, updated app as well as with the clean, newly installed app.

Creating a new profile won't affect the current one, and either one can be launched from about:profiles.

I don't use a Mac, but haven't seen any other reports similar to this one. Since you are working with a new profile, try running OS X in safe mode to check for effects from startup apps and video drivers.

I just did as you suggested and restarted in the Safe Mode. Then, I repeated the steps described earlier with the same exact result. I would like to add that I am a long time, reasonably competent Mac user who does not install any junk apps in my computer and it functions very well, otherwise. I do understand how conflicts can occur as I also volunteer on the Apple user's forum to help solve problems. Since this version was just released, that might explain why there have been no other reports so far. Time will tell. As I said, having someone use a Mac with the current operating system should produce the same result. If not, then it would be a matter of what I have installed that may be conflicting in some way. I do appreciate your thoughts on this issue.

There are other Mac users, a minority, on this forum, who might be able to reproduce the issue, but it would help if you edited the subject of this topic to something like "TB 91/Big Sur hangs on Format - Page colors & background". "Firefox crashes" will be ignored as off topic and too generic.

Thank you for your time and effort to help me. I appreciate it. If as I suspect, the problem is a coding problem with the Mac version, merry attracting the Mac/Thunderbird users will only get others who will offer the other "standard" suggestions, but likely won't resolve the basic issue. I have learned from experience that when an app is bugged with issues, working on the end user's computer merely wastes time. That is currently the case with Apple's latest issue of the Safari browser.

are you closing the mac colour wheel when you do your colour selection. every one of these in recent years I have read has simply been a failure to close the colour wheel so Thunderbird is still sitting there waiting for the wheel to be closed and therefore unresponsive as the last action is still pending.

Yes am closing the color wheel. I wish to make it clear that the last version of Safari worked fine for me and I am not doing anything any different now. Time will show the what I have said is true. This version was released without full testing and it has some programming flaws.

-I selected the choose a color box - from the Thunderbird 'Page Background Colour' window -I then chose a color from the offered choose a color window - from the MAC colour picker -Then Close the MAC colour picker window before doing the following -I then clicked on the OK button in the other window - the Thunderbird 'Page Background Colour' window

The set of instructions you posted, which includes the portion above, sounds like you are leaving the mac colour picker window open when you click on OK in the Thunderbird 'Page Background Colour' window. I've inserted the line that is missing and would be the correct process. Did you close the MAC colour picker before clicking on ok meaning your instructions are incomplete?

If your instructions are accurate, this means the MAC colour picker window has not been closed (after selecting a colour) and is still open when you click on OK in the 'Page Background Colour' window. This is not the correct process, but if it causes a hang/unresponsive etc issue this needs to be known.

It's important to be very sure of the process, so others can test.

Does it also occur if you set the page and background colour first using correct closing of mac colour picker before clicking on OK, in a new Write window before you add any content - that is any text or inserting of image etc?

You asked if I first tried setting the background color? Yes I did. Same problem. For what it's worth. I send out each day an email to a group which has a series of JPEGs inserted. I then manually size them as needed. Sometimes I will insert some typed content. Often I do not. I then center the pictures I've entered. As noted earlier I know to always close the color picker before applying the color. I have been successfully doing this with the earlier version of Thunderbird for years. Let me be clear, these problems only cropped up with the new release. I am not doing anything any different than I have always done successfully.

Also, as noted earlier, it is not possible to enter a second email address into the Address Book.

Thank you for your help efforts, but this is going to turn out to be one of those problems that can't be fixed by instructing me to do something "correctly".

re : As noted earlier I know to always close the color picker before applying the color.

Yeah sorry to cause annoyance - I had to ask for clarity, you may know it and be doing everything correctly, but if someone with a MAC was testing to reproduce by following your instructions explicitly then they would Not have closed the MAC color picker at the appropriate time. I've also noted this occurs in Safe Mode.

I've created a bug report on your behalf for you to follow, but if you feel you can add more info then this is the link: https://bugzilla.mozilla.org/show_bug.cgi?id=1737463

Thank you. I really have loved Thunderbird for years. It definitely has offered features lacking in other apps. I've edited my original input, adding more clarity and I will place that on the link you provided. Your help is appreciated.

A test version is available here: https://bugzilla.mozilla.org/show_bug.cgi?id=1715740#c31

Thank you for providing the link to the test version. I have downloaded and installed it. I am happy to say that, so far, it works perfectly. Once again, Thunderbird is by far the best email client out there, by far.