nbar can cause your browser to hang when pasting an extremely long url, for example view: http://pastebin.com/raw.php?i=dz5vAsKN
The browser will hang for 1 or more minute when trying to edit values of that png data image.
Another frequent bug I witnessed was sometimes awesomebar entries would show up as "null" and stop working. It seems very random which searches it happens with but it happenes enough where it is too annoying. I didn't provide a screenshot but can if anyone is interested.
nbar is a Firefox add-on to help people like me who spend lots of hours working on a computer and they start getting a bit sluggish and might type a wrong URL. At times when you're in a deadline you'll get frustrated if you have to type a URL again and that is why this plugin is handy.
Not only does this plugin does what it offers, but it also runs very smooth and seamless unlike some other plugins.
I've also made a YouTube video about it because it was worth my time: youtube.com/watch?v=AJfs2SNMzm0
All my regards to this plugin developer for coming up with a simple but effective and handy plugin.
Regarding your first point: I have a version which is respecting this idea, but due to some bugs in Firefox I had to change this behavior, otherwise nasty things are happening. There is a workaround for this, I'll probably implement it this weekend.
Referring to its restartless nature: I can say it's quazi-restartless :) - you don't need to restart your browser after installation, but it's required when you disable or remove it. It could be hard to fix because of Firefox's internals. :)
Hope I will find some time this week to repair few things. I'll change this description to something more verbose and informative. The current version version of the algorithm is more or less like this: while you type it searches all possible results distant less or equal to this value ("word distance"). Used metric counts all deletions, insertions, substitutions and transpositions of any two adjacent characters (e.g. distance(tset, test) = 1).