A few suggestions and a bug Rated 4 out of 5 stars

First of all, let me congratulate you for creating such a wonderful addon. It has the potential to become to Firefox Search what the Tab Mix Plus is to Tabs.

Here are a few suggestions:
1. Take a look at an addon called 'Selected Search' (you can't reach the page by searching for it on Mozilla, use the link I have provided below)
It works like your smart search feature but is much better at it because
A) I can more easily have 16 search engines pop up rather than the 7 you provide us with
B) The interface is better and more customisable
C) I do not want a box to pop up for me to enter the keyword of search engine among the smart engines. I instead would like to have all my 16 search engines pop up with their own icon. If I ever want to use another search engine apart from the 16, I type fast enough to go to the address bar or the search bar rather than enter things into the little box you provide us with. So please give us a way to disable this.

2. Add more search engines to the list of search engines you provide us with. Here are a few I'd suggest - Google searches for the last 24hours, 1 week, 1 month, 3 months and 1 year; Google image search without filtering adult content, Google image search for large images; Google search for similiar sites; Urban dictionary; Pirate bay, Drugs.com. All of these search engines are available from the Mycroft project website.

3. Improve the interface of your addon's settings pop up. I think you probably underestimate how unwieldy it is.

4. Your addon does not allow us to use search engine shortcuts more than 3 letters long or shortcuts that contain numerals. I understand that you really might need to keep the 3 letter limit to work with your smart search, but please remove the limitation on using numerals in the shortcuts. I use the Google search for the last 1 day, 1 week, 1month and 1 year a lot, and I had named my shortcuts as g1d, g1w, g1m and g1y respectively, so I couldn't easily import those search engines into your addon. I also use some longer shortcuts like drug for drugs.com, which again I could not import into your addon because of the 3 letter limit, so consider removing this limit too.

1. Image save does not work. I have about 70 addons installed, but none of them should affect any drag and drop functionality except yours. Image save still doesn't work.
2. Can't disable the smart search by leaving the field for search engines empty. The addon keeps showing me an error asking me to either leave the field empty or enter *, when I press save even after emptying the field. (I don't use your smart search, I use selected search instead). I had to go to an older version because of this problem.

Your addon is great. Keep up the good work.

This review is for a previous version of the add-on (2.14.1-signed). 

Well, I don't know what to say - this is the 2nd review in a day's span that someone said my addon's wonderful or so close to perfection, then slap a 4-star rating on my addon. Does an addon have to be perfect and absolutely bug free in all 95 features it offers to get a 5-star rating? I'm beginning to regret spending so much time to offer so much in this addon. Seriously, my extra effort seems to only backfire. Anyhow I hope you can reconsider the rating as did the last person after I made my case and thanks in advance.

Now replies to your suggestions:

1. 'Selected search' addon. Interesting, it does multiple similar things to the Smart Searchbox (even the offset settings!), but it doesn't do preview or text box, both of the latter are very fast search features that's special/unique to my Smart Searchbox - your argument that you could easily go to urlbar and type a search rather than using the text box in Smart Searchbox simply doesn't stand - it wouldn't be nearly as fast, especially since you have to type the selected text again if you were using the 'selected search' addon. The textbox allows quick switching to the exact engine you want, even if you have 80 engines.

Perhaps as importantly, the textbox allows multi-engine search (type 'g,y' searches both Google and Yahoo), FAST driving directions if address is selected (watch demo video), and category search (like type 'Movies' in the textbox to search all movies engines at once) on selected text. This is much more advanced and impossible to do with just icons. You should give these features a try, I can't imagine any user that could not make use of these really useful features, all because of the important textbox.

Still, if that's how you prefer it to behave, you can easily disable Smart Searchbox by following my demo video or FAQ page, then use the 'selected search'. I do not see any advantage of 'selected search' compared to FS except that you could have more icons, which is dubious as it's hard to find icon fast with so many icons popped up. If more users make the same suggestion, I'd consider increasing the number of icons.

2. Add more search engines. One can add those engines from mycroft project page to searchbar then import from searchbar. It takes a bit time, but I don't see why I should pack that many engines for everyone.

3. Unwieldy addon settings. For the sub-settings, it is unwieldy as closing the subsettings does not go back to main Options panel. Otherwise it's actually faster and easier to access the main Options than other addons that uses Addon Manager. I in fact wish all the other addons's Options/Preferences are so easily accessible right from the toolbar. It does take some getting used to though. And if closing subsettings should go back to main Option panel is preferred by more users, I can change this behavior. Otherwise due to technical and time limitations, there's not much I could do.

4. Engine shortcut length and no number limit: These limits exist because: 1. I don't think one likes to type more than 3 letters for an engine, and 3 letters allow a theoretical 17K engines. 2. More importantly, when you type 'drug for fever' in urlbar, how do I know if you want to search 'for fever' on the 'drug' engine you mentioned, or that you meant to a Google search on 'drug for fever'? That's the problem of lengthy and unrestricted shortcuts. I'll probably remove the number limit it in the next version or at most 2.

1. Image save does not work. Do you have a mouse right click drag-n-drop addon (like a mouse gesture addon) installed? I never had issue or see/hear anyone reporting image save not work. If you tried to save a background image or other incorrectly blocked (by javascript on page) images, then you have to enable the 'force save image' in 'Image Save Settings' panel.
2. Can't disable the smart search by leaving the field for search engines empty - yes, you're right. Sorry for my oversight and rest assured it'd be fixed in the next version. Do note that as I stated above, the demo video and FAQ mentioned the preferred way of disabling the Smart Searchbox and it does work. No need to go to an old version of FS.