Firefox ate my longer description of this bug when I attempted to paste a chunk of text -- see https://crash-stats.mozilla.com/report/index/4d74bbb4-5fd8-4021-ab37-ddf582130722 -- but this bug report is because I cannot paste from an emacs buffer on Fedora 18 into Firefox. But the exact behavior is variable -- sometimes I can paste from other apps, sometimes not. (eg cutting & pasting with the mouse from standalone -> Firefox was not working when I started writing this comment, then later was working.) It doesn't seem to be just a matter of the primary selection vs the clipboard working, since if I go through xclip, both work. That's my current workaround: select text in emacs, run xclip in a terminal, paste into the terminal, Ctrl-D, paste into firefox. That has worked every time so far, except when it triggered the above crash. Anecdotally, catlee on irc mentioned that pasting is broken for him too. I thought I had fixed it by upgrading Nightly, but the problem comes and goes.
I see this too. Firefox's X primary selection handling is very flaky and sporadic, and the problem is worst between firefox and emacs. I see it more often in the other direction: doubleclick in the urlbar, go to an xterm or emacs, middlemouse, oops, pasted the wrong thing, go back to firefox, doubleclick in the urlbar again, go to xterm/emacs and this time it works. I have seen flakiness in the direction you describe as well. Unfortunately I haven't found any way to reproduce it reliably or figured out what's different when it does or doesn't work.
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