" ...there are still a number of popular add-ons that weren’t updated for the switch to Firefox 4. People who still have these add-ons installed may be skipping Firefox updates because their add-ons are not compatible yet, and probably never will be. As we make a stronger push to move users away from old and potentially insecure Firefox versions, we need to make sure they don’t lose the add-ons they love.
"Luckily, we have an amazing add-on developer community, where no abandoned add-on goes unforked. I spent a great deal of time looking for the most popular add-ons still in use on 3.6 and lower, as well as their best alternatives. Here they are..."
Source: Mozilla Add-ons Blog
Are add-ons keeping you on Firefox 3.6?