New Version! (19 March 2007)
This is a new improved version. Performance has improved hugely (at least 10x faster normally, and 100x faster on the very longest threads). New features include highlighting of your comments, a less obtrusive layout, highlighting of cortex's comments in Metatalk and an improved list view. Check the screenshot for details.

See a new screenshot. Archived discussion: MetaTalk thread.
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Mefi Navigator does a few things that make quickly navigating Metafilter threads a bit easier:

1. Comment navigation. The left and right arrows allow you to quickly move through a user's comments, while the list icon shows you a clickable list of all his/her comments in the thread. At the top of the list, the '·<' and '>·' allow you to skip to a user's first and last comments, respectively.

2. Admin highlighting. Matt, Jessamyn and Cortex's comments are highlighted in Metatalk only.

3. Original poster highlighting. Especially useful in AskMe threads, to quickly find questioners' responses.

4. Highlights your responses. To find your username, the script reads the Metafilter 'user_name' cookie.
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Works on Metafilter, AskMe and MetaTalk. Works with Firefox 1.5 (not 1.0.7). There's a hack that makes it play nicely with Plutor's Asynchronous Flagging script.

If you have any problems with the script, please email mefinavigator at googlemail dot com.