I'm personally tired of those bloated add-ons which serves no purpose other than providing just a bookmark-style button on the add-ons toolbar. I think the least to say is that these just causes an unnecessary Firefox startup overload if you have a few of them.
So, this restartless and lightweight add-on (4KB code, 8KB package) should make things nicer for those of you who like to use them.
This add-on provides a preferences window where you can define a URL, Image, and Toolbar ID - being the two later optional. If no image is specified the website's favicon will be used instead.
###➜ FAQ ✓###
Q: What is supposed to be in "ToolBar ID"?
A: Some string representing a DOM Node ID, such as nav-bar (default), PersonalToolbar, TabsToolbar, or addon-bar
Q: Reinstalled the add-on and the buttons are missing, even though they show up on the options.
A: Most likely they are gone at the Customize Panel
Q: My button isn't being created!
A: Probably you didn't specified a proper URL (ie, something which contains "://")
Q: Couldn't make it work. all I got was a blank icon.
A: Either you didn't provided an image and the site does not have a proper favicon, or the image you've provided isn't valid - try to specify a[nother] image.
Q: Nice set of default icons!
A: Credit belongs to emey87 for his Trainee IconSet
AMO Listing page: https://addons.mozilla.org/addon/dybutar/