Tech-nut Amit Agarwal explains why and how he uses browser bookmarklets instead of extensions whenever possible to keep Firefox running smooth, then highlights several incredible bookmarklets worth checking out. Agarwal's most compelling reason for preferring bookmarklets to extensions (when possible) is browser compatability:
Add-ins are browser specific, so tomorrow if you decide to move from Firefox to Opera or Google Chrome, your favourite add-ins will no longer work. On the other hand, a bookmarklet written for Firefox is very likely to work with Safari or IE.
I'd add that bookmarklets are easily syncable between browsers using something like Foxmarks, while add-ons still aren't easy to sync. We've covered several of the bookmarklets the post highlights, but they're all very nicely rounded up for easy drag-and-drop "installation" to your bookmarks toolbar. For more bookmarklet fun, check out our 10 favourite bookmarklets.