Update: I managed to log in successfully on my Android phone using the AndroPorts: Thunderbird app. After entering my login information, another window opened with Gmail asking me to authorize third party access for Mozilla Thunderbird Email. Once I did that, I was finally logged in.

This does not help me with my (laptop) computer though. The same problem continues to persist.

re :I'm pretty sure there is no firewall on my computer getting in the way

I would expect a computer to have a Firewall even if it does not have any AV software and Thunderbird must be set up as an allowed program, otherwise the Firewall just blocks it.

I'm using Windows OS and Norton firewall. Thunderbird will only work if the Firewall has TB as an allowed program.

So make sure TB is allowed or maybe you need to allow the ports.

re :When I provide the name/email/password, and select IMAP, the menu hangs for a long time (minutes) saying "Checking password..." Then it finally gives up and says "Configuration could not be verified -- is the username or password wrong?"

provide the name/email/password and select checkbox to remember the password. Click on 'Continue'

Click on 'Manual Config' button check imap is correctly chosen. Check username is full email address. check the 'Authentication Method is 'OAuth2' click on 'Done' then gmail should ask for verification via browser to confirm you are you. Then an OAuth2 token is created and stored. From then onwards that token is used instead of normal paasword.

I would expect a computer to have a Firewall even if it does not have any AV software and Thunderbird must be set up as an allowed program, otherwise the Firewall just blocks it.

I'm using Windows OS and Norton firewall. Thunderbird will only work if the Firewall has TB as an allowed program.

So make sure TB is allowed or maybe you need to allow the ports.

I researched this issue further for Arch Linux and I cannot find any documentation suggesting that Thunderbird needs to be explicitly allowed. I also can't figure out what rule I would create to allow it.

then gmail should ask for verification via browser to confirm you are you.

I have done all the steps leading up to this point -- it is this verification step that is not being generated. This is when Thunderbird just hangs at "Checking password" for minutes. Is there something besides firewall that could be interfering at this step?

If you're already using OAuth and enabled less-secure apps, the only other factor I can think of that would stop the setup is if you don't allow cookies for *.google.com in Edit/Preferences/Privacy in TB. It might also depend on cookies allowed in the browser.

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

My problem got solved but I have no idea what fixed it. I'll explain what I did anyway.

- Downloaded ufw and gufw (firewall configuration packages) to try to allow Thunderbird explicitly -- no help here.

- Downloaded firewalld to try the same -- again no luck.

- Power off computer.

- Had to go to university and physically be in a different wifi network. Now, laptop on boot cannot even start the wireless interface / netctl service.

- Tried resetting iptables to its default values -- no help.

- Scoured Google, eventually ended up deleting all my frequent netctl profiles.

- Reboot and am able to use wifi again.

- Tried out Thunderbird and magically the verification window finally popped up like it was supposed to.

The most sensible difference I can come up with is that for some reason the different wifi network did the trick. I'll see when I get home if TB breaks again. Thanks for the assistance.