Somewhat slow response Rated 3 out of 5 stars
Agreed with last reviewer...this does too much in one add-on. I find it can run very slow (on an iMac 24" 2.8GHz w 4GB, so this CPU is no slouch).
I would at least split the function to highlight and show like Safari as a separate add-on from the Language search facilities.
I find that after a while, the Safari-like features get a little confused and the Find field does not respond to the keyboard well. Hiding the Find field arbitrarily shows/does not show the search result locations. Type to start find sometimes no longer responds.
Otherwise, this is the only add-on I have found to provide these functions.
Combine this with the Find In Tabs add-on and this is a killer combination.
Because new useful APIs are landed, most of performance problems around the find toolbar will be gone on Firefox 4.0. If possible, try Firefox 4 beta with XUL/Migemo 0.13.3.