This wouldn't seem very useful except that effectively what the whole shebang does is convert a compile-time error into a runtime warning, such that other functions that previously might not have been able to load because of the error can now be parsed and hopefully run. With luck this should improve the functionality of sites using these functions even if everything still doesn't fully work, as a down payment hopefully on a future implementation. It may not be technically possible but it's a start.
Which reminds me, and since this blog is syndicated on Planet Mozilla: hey, front end devs, if you don't have to minify your source, how about you don't? Issue 533, in fact, is entirely caused because uglify took some fast and loose shortcuts that barf on older parsers, and it is nearly impossible to unwind errors that occur in minified code (this is now changing as sites slowly update, so perhaps this will be self-limited in the end, but in the meantime it's as annoying as Andy Samberg on crack). This is particularly acute given that the only thing fixing it in the regression range is a 2.5 megabyte patch that I'm only a small amount of the way through reading. On the flip side, I was able to find and fix several parser edge cases because Bugzilla itself was triggering them and the source file that was involved was not minified. That means I could actually read it and get error reports that made sense! Help support us lonely independent browser makers by keeping our lives a little less complicated. Thank you for your consideration!
This version also "repairs" Firefox Sync support by connecting the browser back up to the right endpoints. You are reminded, however, that like add-on support Firefox Sync is only supported at a "best effort" level because I have no control over the backend server. I'll make reasonable attempts to keep it working, but things can break at any time, and it is possible that it will stay broken for good (and be removed from the UI) if data structures or the protocol change in a way I can't control for. There's a new FAQ entry for this I suggest you read.
Finally, there are performance improvements for HTML5 and URL parsing from later versions of Firefox as well as a minor update to same-site cookie support, plus a fix for a stupid bug with SVG backgrounds that I caused and Olga found, updates to basic adblock with new bad hosts, updates to the font blacklist with new bad fonts, and the usual security and stability updates from the ESRs.
I realize the delay means there won't be a great deal of time to test this, so let me know deficiencies as quickly as possible so they can be addressed before it goes live on or about September 2 Pacific time.