Regressed by bug 1413550 fixed by export bug 1415812. https://hg.mozilla.org/mozilla-central/rev/b4d1db669c93

gchang says this bug has missed 58.0b8. It doesn't have a patch with beta approval flag. Please attach the appropriate patch without install.rdf.in changes and request approval ASAP.

Comment on attachment 8933357 [details] Bug 1421312 - Import buttons on activity Stream incorrectly wrap. Approval Request Comment [Feature/Bug causing the regression]: https://github.com/mozilla/activity-stream/commit/73ea6dc43a149fb9a31a0b23cd03d6a5b97d13e5 [User impact if declined]: minimal [Is this code covered by automated tests?]: no [Has the fix been verified in Nightly?]: yes [Needs manual test from QE? If yes, steps to reproduce]: incorrect button wrap on lower resolution devices. no steps required. [List of other uplifts needed for the feature/fix]: [Is the change risky?]: no [Why is the change risky/not risky?]: not risky. minimal css change. [String changes made/needed]: none

Comment on attachment 8933357 [details] Bug 1421312 - Import buttons on activity Stream incorrectly wrap. https://reviewboard.mozilla.org/r/204270/#review209884

Comment on attachment 8933357 [details] Bug 1421312 - Import buttons on activity Stream incorrectly wrap. Fix an import buttons incorrectly wrap issue. Beta58+.

I managed to reproduce the initial issue on 59.0a1 (2017-11-13). I also can confirm that 59.0a1 (2017-12-28) and 58.0b13 build1 (20171226085105) are verified fixed using Windows 10 x64, Windows 7 x86, Ubuntu 16.04 x64 and macOS 10.13.2.