Just a quick note to Firefox nightly testers and bug triagers: I pushed a patch that makes Firefox no longer honor the html5.parser.enable pref. (It is up to the product drivers to decide if the patch lands on Aurora and Beta, too.)

The pref was originally created as a means to test the HTML5 parser before it was ready to be turned on by default. After Firefox 4 was shipped with the HTML5 parser enabled, the pref was kept around as an easy means for testing if a regression was caused by the new parser. However, it was all along the intent that flipping the pref to false was an unsupported configuration available for regression testing.

Unfortunately, there have been folks out there advising Firefox end users to flip the pref. For example, one major company appears to have been telling their customers to flip the pref until the company got around to fixing their extranet to be compatible with the HTML5 parser. Chances are that end users who have received advice to flip the pref for whatever reason are not going to take care of flipping it back.

In the interest of ensuring that end users do not run Firefox in an unsupported configuration, the pref is now gone ahead of the old parser code.

(The old code is still around for various dependendencies around the code base. The dependencies include View Source, Netscape bookmark import, about:blank, plain text conversion for the clipboard and a couple of Thunderbird message sanitization and conversion functions. Changing those dependencies has proved slower than expected.)