User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0 Build ID: 20160909004004 Steps to reproduce: Currently, firefox hyphen line break behavior is nonsensical and not doing what the docs say: ­ works as expected. It inserts a line break opportunity the browser clearly uses. However, https://developer.mozilla.org/en/docs/Web/CSS/hyphens clearly states U+2010 / ‐ / ‐ is supposed to be a visible line break opportunity. ... except that firefox completely ignores it and only breaks at ­ points which is super silly and inconsistent, and then it even renders ‐ and ­ line break hyphens _completely different_ in some occasions, breaking the expected hyphen support even more. Are people supposed to only be able to line-break words where there AREN'T actually dashes in the word? So "hands-on" can't have a reasonable line break? If not, this should probably be fixed.

TL;DR: docs say hyphens work like ­ but they simply don't do that at all, and to add insult to injury they even look completely different for no obvious reason.

The previous title was quite judgemental, I changed it. Sorry about that. I had spend a while reading the docs and frustrating myself with line break behavior, but that's still not an acceptable reason for bad tone in bug reports. Also, if I just misread the docs somehow or made a mistake (which is easily possible) and this isn't actually a bug, feel free to tell me about the silly things I have done :-)

I can reproduce this. It does not break-line when words contain dashes as mentioned by the reporter. -- Version 51.0a1 Update Channel nightly User Agent Mozilla/5.0 (X11; Linux i686; rv:51.0) Gecko/20100101 Firefox/51.0

This is the same issue as described in bug 809020. More specifically, it does not break at explicit hyphens if they occur within 6 characters of the start or end of the word. If you have a sufficiently-lengthy hyphenated word (like that!) then the hyphen _will_ be used as a potential break point. This happens because when we're within the "conservative" range here: https://dxr.mozilla.org/mozilla-central/rev/17242152a8fbe65f20b864bd5a577b0dff624857/intl/lwbrk/nsJISx4051LineBreaker.cpp#614-622 hyphens are not seen as breakable. This is also the cause of the odd behavior reported in bug 745517 (see the testcase there).