Meh. Yeah it looks like a Linux port is still in the works (which is funny because lipcap is a Linux library). This extension isn't pure JS like many other extensions. It should also be noted that this is far from useful due to it's many limitations.

For those interested in the "real deal":
Ferret+Hamster+sslstrip+arpspoof.
http://hamster.erratasec.com/ the compile options are, well pretty strange, read the README. I've compiled it for the N810 in the past, shouldn't be any issues there. However to actually utilize this tool you need a patched version of arpspoof compiled (dsniff package). This is the major roadblock I had, IIRC the issue stems from not being able to override libnet2 with libnet1 in scratchbox (plus you need to change the hardcoded eth0 to wifi0 or use the patched code that is floating around somewhere, I'll have to look, it's hard to google for it). Then you need to get ssltrip working which requires some extra Python components which I simply couldn't find.
When it works, it WORKS. When it doesn't, it knocks out my router, heh.
gmail was completely vulnerable last time I checked and Facebook works if you have checked the box to stay signed in (uses a cookie).

Basically how this works is you tell the router that you are the IP of the victim and you end up stealing their cookies. However at no time does the attacker have access to the actual password. When the victim signs out, this nullifies the cookie and signs the attacker out too. This is one of the better pen-testing exercises IMO (if you can get it working properly; I may have forgotten to mention that this doesn't work on a WPA2 network).