Did some further testing on a completely different computer, and got steps to reproduce this at least in my case:
Windows 7 Pro x64
Thunderbird 38.1.0
Left on "Standard" install.
Unchecked use Thunderbird as my default mail application
Launched Thunderbird
Unchecked Email default client option. Unchecked Always perform this check when starting Thunderbird.
Selected "skip integration"
Selected "I think I'll configure my account later"
Selected "Keep" regarding Lightning extension prompt.
Closed the open source message.
Tools->Addons->Extensions
Checked extensions: Lightning 4.0.1 is included with Thunderbird 38.1.0
Left that as is.
Get Add-Ons->searched for "provider", installed Provider for Google Calendar 1.0.4.
Clicked Restart Now link.
Selected "I think I'll configure my account later"
Closed Addons Manager
Pressed Alt to get menu.
File->New->Calendar->On The Network->Google Calendar->Next
There are no accounts listed already on this screen now.
entered one of my @
gmail.com email addresses
Next
Prompted for OAuth credential/permissions.
Logged in, clicked Accept button.
Selected all Calendars(Email account+Birthdays+Holidays in Canada+My name's Task List)
Next
Finish
Exited Thunderbird.
Started Thunderbird again.
Selected "I think I'll configure my account later"
No issues with a repeat OAuth Prompt.
Added my email account(same account I'm using with the calendar):
Alt for menu
File->New->Existing Email Account
Entered email address
Entered password
LEft Remember Password checked.
Continue
Left IMAP selected. Clicked Done.
Prompted for OAuth credential.
Signed in there.
Clicked Accept
Interesting. Thunderbird appears stuck at "Sending LOgin information" for Mail for
mye...@gmail.comClosed Thunderbird and reopened it.
Loaded email fine. Seems okay? No OAuth prompts.
Restarted Thunderbird again.
No issues, no OAuth prompts.
Switched to Calendar tab
Clicked Synchronize button.
No issues, no OAuth prompts.
Closed Calendar tab.
Alt for menu->Tools->Options
Security Tab->Passwords Tab->Saved Passwords
There are 4 saved passwords here:
1. IMAP
2. Google Calendar OAuth
3. OAuth for
accounts.google.com4. SMTP
Clicked Remove All, confirmed Yes, clicked Close, clicked OK.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Prompted for OAuth for Google Account.
Entered credentials again, signed in. Clicked accept.
Restarted Thunderbird again.
No issues, no OAuth prompts.
Alt for Menu->Tools->Addons
Searched for keefox
Installed Keefox Keefox 1.4.4.1-signed (This machine already has Keepass 2.30 installed.)
Clicked Restart Now link.
Prompted for OAuth for Provider for Google Calendar (Interesting!)
Provided credentials, signed in, clicked Accept
Switched to Extensions tab out of curiousity; Installed extensions: KeeFox, Lightning, Provider for Google Calendar
Closed Add-ons Manager
Restarted Thunderbird.
Prompted for OAuth for Provider for Google Calendar
Provided credentials again, signed in, clicked Accept
Alt for Menu->Tools->Options
Saved Passwords
2 saved passwords here:
1. Google Calendar OAuth Token
2. OAuth for
accounts.google.comClicked Remove All, confirmed Yes, clicked Close, clicked OK.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Prompted for OAuth for Google Account.
Entered credentials again, signed in. Clicked accept.
Clicked Get Messages. seems fine...
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Alt for menu->Tools->Addons
Disabled Keefox
Clicked Restart Now link
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Removed Keefox.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Entered credentials again, signed in. Clicked accept.
Quit Thunderbird again.
Restarted Thunderbird again.
Prompted for OAuth for Provider for Google Calendar.
Closed window, uninstalled Thunderbird.
I'll point out as well, I've previously used gContactSync(Which also uses Google OAuth) for a long while without issue with Keefox. Provider for Google Calendar is the new kid on the block for me.
From the looks of this testing, if I had to guess, I'd say there's probably something that gets messed up with Provider if another extension touches how Passwords/OAuth are stored? As you can see, it doesn't seem to get fixed even after uninstalling the other extension, either. (Note in this test Keefox wasn't actually setup to *do* anything, it was just installed, and probably has some sort of handler for how Thunderbird deals with passwords, even if it's not being actively used: it'll change the login prompt for non-OAuth credential gathering in Thunderbird to have a Launch Keefox button, and will auto-fill passwords if it's been linked with Keepass and Keepass is running. Note this does not happen with OAuth credentials, just normal passwords.)