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Okay, this thread prompted me to finally get around to trying out the ProtonMail bridge, which you can only use with a paid account.
And, over two hours later, I can finally report on the results.
First, the bridge help documents are good, but really annoyingly designed, so you have to click next for each step. That said, the URL you'll want for that, is:
Install the bridge.
Run the bridge.
Choose OK in the dialog which opens. This dialog is weird: for me it didn't read or even focus automatically, but I was able to alt+tab to it, and read it with object nav. It just tells you to add an account, so it's safe to select OK.
After selecting OK, you are in a probably non-speaking window. Use object nav to move to the right (NVDA+numpad 6), until you reach "add account", and select it with NVDA+numpad enter.
You are then in a field for entering your ProtonMail email address. Enter it, press tab, and enter your password.
The instructions say you may also have to enter a second password, or 2FA code here.
Then press tab again twice, to "next", and press it.
Once again, nothing talks after this. You are hopefully in a window that says that you have successfully added your account, object nav again to the rescue.
You can probably choose OK or close here, either way you should return to the main window.
The instructions say to click on mailbox config, to learn how to connect. However there is no mailbox config evident on the main screen when tabbing around.
To find it, tab to your address, and again use object nav. Previous object says "expand row". I'm not sure if you need this, but I did it.
After that (assuming that was necessary), next object will give you options such as: switch to split address mode, and mail configuration.
If you are going to switch to split address mode, I suggest doing it before you set anything up in your mail program.
Here is where the most inaccessible part arises. Pressing mail configuration, which you need to do to get your access information, seems to only result in a "help balloon", which you can only get to with alt+tab, although actually you can't read it at all, even though it claims you have switched to it.
I tried everything I could think of to get there and read this, but could not.
It was after switching to Narrator, that I realized that Windows Defender was blocking features of the bridge. After allowing the bridge, I switched back to NVDA.
I still could not get object nav (or any other kind), to work in the window that was displaying (the mail configuration window).
However, by going back to the desktop, I was able to use object nav to go to the parent object, then the next object which was the window with my email address as its name.
After that, I next objected through the window menu (close, minimize, etc.), and then the next thing I knew I was hearing the host name (127.0.0.1), the IMAP port (1143), and all the other information! (SMTP port is 1025.)
Each element had its own copy to clipboard button. I thought the copy button would be especially useful for the password, but NVDA+C reported nothing copied.
Interestingly enough, however, I was able to use NVDA+F9 to mark start, review cursor right by character (numpad 3) to the end of the password, and then NVDA+F10 to mark end, and thereby copy the password to the clipboard.
The bridge requires starttls in your mail program, and runs on unusual ports, but otherwise uses normal SMTP and IMAP. They have help pages on configuring all of the common mail apps.
I could not find any way to get out of the Mail Configuration part of the bridge, other than by closing and restarting the bridge.
I hope this all helps someone.
Please, to anyone using ProtonMail, write to their support and ask them to improve the accessibility of the bridge!