Sticky connections and X-Forwarded-For
I'd like to know if the "Use sticky connections" parameter is able to use the IP provided in the X-Forwarded-For header added by squid instead of the IP source address (the squid/pfsense itself)
For info, I'm currently using pfsense 2.2.1 nanobsd i386
No. Sticky relies on pf state/session tracking, it does not inspect the contents of packets, and likely has no bearing on squid whatsoever.
If you need balancing help with squid, check the Cache/Proxy subforum under Packages.