useful but avg user may not need it Rated 4 out of 5 stars
Firefox already respects Strict-Transport-Security, maintaining an internal list of sites that declare that the browser should always use https to access them. This prevents someone from spoofing as this site (man in the middle) but with plain HTTP. It also has the added benefit of simply forcing encrypytion to protect your data in case you tried to visit the plain HTTP site (particularly from a public/open hotspot) -- much like the Electronic Frontier Foundation's "HTTPS-everywhere" (eff.org) add-on except that the latter uses list maintained by the EFF rather than as self-identified by each site.
The benefit of the Force TLS add-on is that it provides a nice GUI for displaying and modifying Firefox's list of STS sites -- although the usefulness of this is limited by the limited number of sites that set a Strict Transport Security header.
Also, one can easily add a site in the GUI, whereas HTTPS-Everywhere requires hand-editing arcane xml for the user to add a new site!
HTTPS-Everywhere on eff.org has the sizable aforementioned list of sites that *can* use HTTPS in place of HTTP, even if the sites don't themselves request that the browser always do so. I guess HTTPS-Everywhere can use more flexible rules, but again at the price of requiring you to write XML to define each of them. Also HTPPS-Everywhere provides a button showing any entries for the current site and the ability to disable/enable these entries (but not add a new one).
You might try using *both* of these add-ons together. Perhaps, in the future, one of these add-ons could provide the benefits of both.
However, this addon may not be needed by an avg user who doesn't want to know what's going on at this level of detail. For him/her, the HTTPS Finder addon might be a better choice, perhaps in combination with HTTPS Everywhere. HTTPS Finder tests whether the present HTTP site has an HTTPS version having the same URI, and offers to switch you to it.