Bugzilla

John K.
I'm assuming you know how to find and install the 78 version of tb on mac. Since you have already installed and run 91, if you try to run 78 you will be requested to create a new profile to continue. You can do that if you want but you will have to download all the messages in the accounts again. So to avoid this, you need to use the --allow-downgrade option on tb 78.

You can run the installed tb 78 by going to "Utilities" and running the "Terminal" app.  In the terminal app you can run thunderbird like this:
```open /Applications/Thunderbird.app  --args  --allow-downgrade```.

There seems to be no "easy" way to add a command line option to the TB icon you normally click to run TB on macOS/OSX. So you have to run tb from a terminal. 
There may be other ways but it involves writing "scripts" and running them as apps, but this seems to be the easiest for just a temporary use.
Re: https://superuser.com/questions/16750/how-can-i-run-an-application-with-command-line-arguments-in-mac-os

John K.
I'm assuming you know how to find and install the 78 version of tb on mac. Since you have already installed and run 91, if you try to run 78 you will be requested to create a new profile to continue. You can do that if you want but you will have to download all the messages in the accounts again. So to avoid this, you need to use the --allow-downgrade option on tb 78.

You can run the installed tb 78 by going to "Utilities" and running the "Terminal" app.  In the terminal app you can run thunderbird like this:
```open /Applications/Thunderbird.app  --args  --allow-downgrade```.

There seems to be no "easy" way to add a command line option to the TB icon you normally click to run TB on macOS/OSX. So you have to run tb from a terminal. 
There may be other ways but it involves writing "scripts" and running them as apps, but this seems to be the easiest for just a temporary use.
Re: https://superuser.com/questions/16750/how-can-i-run-an-application-with-command-line-arguments-in-mac-os

Edit: You can get the last 78 versions here (Assuming en-US locale): https://archive.mozilla.org/pub/thunderbird/releases/78.14.0/mac/en-US/
I installed from file my addon in comment 169 and ran it in terminal with "open /Applications/Thunderbird.app --args --allow-downgrade" and it worked OK with v78.14 when I copied a folder to an icloud account. Actually, I had never tried running the addon with macos until now.
FWIW, on this old macbook air with "mavericks" the os is too old and tb 91 won't install but 78 still does.