IRUnfilterIRUnfilter
This add-on is based on the theory described in my blog at www.abiusx.com, which is in Persian.
The post describes how Iran's content filtering searches only the "Host" HTTP header for filtered website URLs, so it removes this header.
It also switches itself off in HTTPS connections, since HTTPS requires the Host header.
Also supports automatic switching over to web proxy (enabled via its button in the toolbar), for those sites where IP addresses are filtered and you can't connect.