User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

TB 91.2.0 (64-bit)

I do not want to be TB able to connect to the internet. Because of that, I have set the http(s) and socks host in "Configure Proxies to Access the Internet" to sockets where nothing listens, so it's a dead-end and only exclude the mail servers and openpgp key server from that (see attached screenshot).

$ telnet 192.168.1.1 18118
Trying 192.168.1.1...
telnet: Unable to connect to remote host: Connection refused
$ telnet 192.168.1.1 19050
Trying 192.168.1.1...
telnet: Unable to connect to remote host: Connection refused

For TB Updates I have set "Check for updates, but let me chose whether to install them" (see screenshot).

Actual results:

• TB has informed me that a new TB version 91.2.0 is available.
• when I click on "check for updates" it says "Thunderbird is up to date"

Expected results:

• The Updater should not work because it cannot reach the internet
• When the Updater is used it should either silently fail or produce an error message like "Could not check for updates: Internet access error"
• In any case it should not say "TB is up to date" because it should not be able to decide that

If this works, then it probably is a bug in the core Mozilla network engine code.

What was the behavior with TB 78 ? Was it blocked?

Ancient bug - the dialog is just not being super helpful.

The old subject was correct. As I have written in OP "TB has informed me that a new TB version 91.2.0 is available.".

That was with I think TB 91.1.0. I clicked "Install update", and TB updated itself from 91.1.0 to 91.2.0 then.

So the whole Updater is working, including the Checking for Updates, Downloading and doing it.

The problem is I guess that the set http(s) proxy is not used.

Not sure what this has to do with Offline-Mode that is subject to bug 800321.

(In reply to Kai Engert (:KaiE:) from comment #3)

What was the behavior with TB 78 ? Was it blocked?

I cannot say, I used TB 78 from the Debian Repository and did not use the TB internal Updater.

Only since I was curious about TB 91 and because I need to run the newest release before reporting bugs (to confirm the bug still exists with the newest release) I switched to the manual downloaded TB 91 Tarball and then first since years used the TB builtin Updater, where I stumbled about this bug.

With 94.0b2 and having http(s) proxy set to dead-end like in OP, today I received update notification for a new version.

But when clicking "install update" there came the message "TB cannot update to new version" and I was redirected to thunderbird.net.

But after setting proxy to "no proxy", so using direct internet connection, and then clicking again on the button "install update", the TB internal update to version 94.0b3 happened.

So it looks to me like the download/update itself now honors the http(s) proxy setting, and only the check for update still ignores it.

I've learned this is per design - some add-ons were abusing the proxy system preventing the application from updating properly.

See https://blog.mozilla.org/security/2021/10/25/securing-the-proxy-api-for-firefox-add-ons/

"Starting with Firefox 91.1, Firefox now includes changes to fall back to direct connections when Firefox makes an important request (such as those for updates) via a proxy configuration that fails. Ensuring these requests are completed successfully helps us deliver the latest important updates and protections to our users"