STR: open attached file, select Tools -> Web developer -> Page source (Cmd+U). Actual results: the page source is empty for a few seconds, during which Firefox uses 100% cpu doing reflow. Expected results: view source should display as fast as in Firefox 14. Regression window points to bug 751841: Good: http://ftp.mozilla.org/pub/mozilla.org/firefox/tinderbox-builds/mozilla-inbound-macosx64/1338359573/firefox-15.0a1.en-US.mac.dmg (built from http://hg.mozilla.org/integration/mozilla-inbound/rev/b613ac5ff64a) Bad: http://ftp.mozilla.org/pub/mozilla.org/firefox/tinderbox-builds/mozilla-inbound-macosx64/1338360714/firefox-15.0a1.en-US.mac.dmg (built from http://hg.mozilla.org/integration/mozilla-inbound/rev/e98f0fc3d02e) According the the built-in profiler, with the patch we spend 4.5 seconds on my machine under PresShell::DoReflow, whereas before the patch it was 1 second. I tried to simplify the page and I see the slowdown with 1) a long line with entity references (") 2) many lines, with or without entity references

This possibly needs to be split into two bugs, something like a) performance is bad with lots of elements with unicode-bidi: isolate; b) view-source uses lots of elements with unicode-bidi: isolate. This is a patch for b), i.e. it fixes the issue with view-source slowdown, but there could be similar slowdown in other cases.

I have *no* clue where that first hunk is coming from :(

I went ahead and landed the patch: http://hg.mozilla.org/mozilla-central/rev/42f8002ca13b

Thanks for the quick fix! The issue is now fixed for me.