A few problems ... Rated 5 out of 5 stars

1.) The current version (2.23) states on addons.mozilla.org that it's compatible with Firefox 11.0+. However the extensions's install.rdf specifies 12.0 as the minimum Firefox version. :-( Thus if you install this on Firefox 11.0, it instantly gets disabled due to the min. version check.

2.) If you override the version requirement (eg. download the XPI, extract it, modify the install.rdf, repack, install), the extension crashes Firefox right when you hit Ctrl+Shift+F to activate it's search bar.

It'd be nice to have these fixed. Thanks.

P.S.: the above observations came from a PC running Ubuntu 10.10 (Maverick) and Firefox 11.0. I used a new/clean profile with no other addons.

3.) On Windows XP + Firefox 13.0.1 the extension works as advertised. Small bug: even if "disable all instant ..." is checked and regex search is checked, the find-as-you-type seems to be enabled. At least for me it did search and highlight expressions, when I typed a regex (and enabled regex search and disabled all instant features). Not that I'm complaining (it's great that find-as-you-type works even under these circumstances) ... it's just a mere observation that it doesn't work as the documentation (I mean inline help) described it.

Update (June 20, 2012)
@Mingyi: thanks for the quick reply. With your fixes it's now quite fine. :-) I've modified my rating to five stars.
I've one suggestion though (an enhancement request): the search bar/field appears over the current page and hides a portion of the page. Thus if a match of the expression is found (and highlighted) "under" the Fastest Search box, it's a bit hard to see. It'd be nice if the Fastest Search box would not appear in the current page, but as a separate search field (part of the browser window ... as the builtin search field does). Of course I fully understand that this would mean a whole different approach and a complete rewrite ... but it'd be nice. :-) There're lots of extensions that open their own "section" below the main browser content (eg. Firebug, HttpFox, just to name a few), you can check them for an example on how it can be done.

This review is for a previous version of the add-on (2.23). 

See update at end please.

1. My bad stating it's compatible with v11 - I just updated to say minimum is 12. Please update your FF to 12 or 13 (which is the current stable version), OR use Fastest Search v2.22 with FF11, if you must use FF11. BTW - it's not my extension crashing FF, it's the Addon SDK that my extension uses is NOT compatible with FF11. I did not realize that the use of the newer SDK (despite that the API I used was old) would crash FF11 when used.

2. Disable all Instant just disables instant page loading of history/bookmark/Google results in urlbar and searchbar, it's unrelated to FAYT, which does text search on already-loaded pages. I did notice that you were right pointing out that my inline help said FAYT is disabled when you type regex in the Search Box, but it actually isn't. Indeed, in much older FS versions I disabled FAYT in Search Box for regex for some reason that I later overcame. So regex FAYT is enabled in Search Box now but I forgot to update the docs. I just updated it and will release it with the next version.

Update: thanks for updating the rating! As for your suggestion, indeed that'd be a big rewrite. There are two ways I tried to tackle this issue - none perfect: 1. You can choose to "Use small search panel" in "Text Search Settings", which is much smaller. 2. You can press the "Search" button to dismiss the Search Box. Since Fastest Search remembers your keyword, you don't lose much by dismissing it temporarily. Fastest Search is Addon SDK based, which makes it hard to use XUL directly - although I figured it out quite some time ago. Still the rewrite would take much effort. Let me think about it and see if there's a better workaround.