Roboform addon is 331 kb whereas LastPass is 1.8MB in Firefox
This is a specious argument -- you have to download and run other software in order to support the 331kb Addon. It's absolutely useless on it's own. LastPass at 1.8M can (and does) run entirely independently and isn't running if you close Firefox. The same holds true when you try to compare memory or CPU time -- you have to add up all the pieces running to make a proper comparison.
Please note that I'm biased so take what I say with a grain of salt.