Rated 4 out of 5 stars

I really only needed this for the regex searches - I am completely happy with InstantFox and prefer to use that. This needs an options to completely disable sections, such as disabling everything to do with the URI bar. Disabling Instant is not everything - it quite clearly still modifies the URI bar when InstantFox is off. Also, maybe use a more standard options box, this one is rather hard to use.

Aside from that, the regex search functionality is excellent.

My rating is based on my user experience, I have no doubts this addon is capable of doing all it says very well, and it is likely perfect for the majority of users.

This review is for a previous version of the add-on (1.98). 

Fastest Search's Instant is at least as powerful and flexible than InstantFox's, AND FS instantly search bookmark/history too by default (can be turned off). I found the bookmark/history Instant the most-used Instant I myself use. That's not to mention so many other features provided by FS. I don't see a reason why one'd want to install both FS and InstantFox.

Regardless, FS also has a very obvious option in the main Options panel, with partial red font that says "Disable All Instant Features". I can't find any Instant feature still working after this red-font option is checked. I just realized that your saying FS "still modifies the URI bar when InstantFox(sic) is off" is probably about the Suggest box still on when you use engine shortcuts, right? That's of course to be expected - turning off Instant does NOT mean that user wants to turn off Suggest too. If needed, just turn Suggest off too.

As for Options panel, yes, XUL preference system is a better menu system. But:
1. it's not my fault - Jetpack (restartless) addons cannot have those Options boxes (not until very recently and it might still not be available fully, and even if so, it's too much to move my myriad of options over).
2. FS's Option panel's easy accessibility actually makes setting changes faster than XUL system.
3. The context-sensitive help "?" FS provides is also better than the VAST majority of other addon's Options.

FS does have a lot of features (maybe a bit overwhelming), which makes it more favored by users willing to spend a bit time exploring/learning. It not only fills in the void not filled by other addons, but also replaces similar addons that do less. Give FS some more time, I'm sure you'd find it better than others and feel no need to keep other similar addons. If so I wish that you consider revise your rating.