I cannot see what is happening when attempting to upgrade on the admin webui on latest Firefox.
Could you confirm if your site successfully upgraded after clicking the upgrade button, even though the text that would typically be displayed in the black box was missing, or if the site failed to upgrade at all?
I tried to reproduce this on Firefox version 91.11.0esr (64-bit), while upgrading from Discourse v2.5.0.beta5 to 2.9.0.beta7, but didn’t encounter any errors during the upgrade.
What version of Firefox were you using, and what versions of Discourse were you trying to upgrade from?
Yeah, the webui update totally failed for me on both Firefox and Chromium. Went back to using command line tools and all I had to do was
reload app. Everything works again and update worked without issue.