In Nightly after 2017-05-02 is the tab bar broken / partly hidden when "Tab Groups" is enabled. I have also filed an issue (with screenshot) over at the add-ons Github repo: https://github.com/Quicksaver/Tab-Groups/issues/541 The developer noted in the add-on's "About" page that the update from January 27, 2017 was the last update[1]. Therefore it seems little likely to me that this will be fixed for 55/56 from his side. I was hoping that someone could have a look at the regression range and see if something can be (un-)done on Firefox's site. [1] http://fasezero.com/lastnotice.html

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:55.0) Gecko/20100101 Firefox/55.0 Firefox: 55.0a1, Build ID: 20170508030204 I have managed to reproduce this issue only on Mac Os on latest Nightly (55.0a1) build. I have performed a regression range and here are the results: Last good revision: af0598e94338f6fb803a3dec0f56728ce179f954 First bad revision: cf470e6222f971b5cb4551ff627b49152a326c22 Pushlog: https://goo.gl/xw1tJR Looks like the following bug has the changes which introduced the regression: https://bugzilla.mozilla.org/show_bug.cgi?id=1352364 Johann, can you please take a look at this?

Thanks for moving, dao. This is not something we will undo on Firefox side. We consolidated our toolbar styling to be the same across all three platforms, so this shouldn't be hard to fix on add-on side by changing their code to use the CSS they use for Windows.

There is a workaround here while the add-on is still working: https://github.com/Quicksaver/Tab-Groups/issues/541#issuecomment-309524788 The add-on developer announced already that he will not update his code anymore; closing this as wontfix