Mozilla/5.0 (X11; Linux x86_64; rv:63.0) Gecko/20100101 Firefox/63.0 ID:20180807100107 When I drag the Slack tab into its own window the pound sign icon is replaced by the generic slack icon. If I reload the page, the pound icon comes back. This is not happening with 61.0.1 I put a screencast of the bug here: http://video.chevrel.org/faviconBug.mp4

I just tested and was able to reproduce this on Firefox 61.

Working here in 61 but it's the distro build, I'll test with a mozilla build to make sure this is not a recent regression. I actually filed this one because I thought it might be a regression from bug 1453751

Used mozregression to narrow down to this change: https://hg.mozilla.org/integration/autoland/rev/266a3c9eae61 Bug 1449523 - ContentLinkHandler only picks perfectly sized icons. r=Mardak

This looks like an example of the problem described in bug 1478823 comment 3. Basically we do different things depending on the timing of encountering <link> tags. When slack loads it defines some default icons then sometime later adds a new link icon which we then consider to be the most important. When we detach the tab we re-fire all DOMLinkAdded events for the detached tab (described here but I can't figure out where this is triggered from the C++! https://searchfox.org/mozilla-central/source/toolkit/content/widgets/browser.xml#1336). Since they are triggered at the same instant our code for determining the best favicon is working with a different set and so gives a different result.

Too late to fix in 63, but we could still take a patch in 65 and potentially, 64 beta.

Permalink for comment 4: https://searchfox.org/mozilla-central/rev/a7f4d3ba4fbfe3efbde832869f1d672fce7122f6/toolkit/content/widgets/browser.xml#1695