Hello,the current firefox 3.5 version that is available is actually the stable version of 3.5 and not a beta. Here is an extract from clems blog.
Don’t mind the fact that it’s called “Shiretoko” and that it mentions “Beta”, this version of Firefox is the stable 3.5 version. Upstream, Ubuntu decided to keep version 3.0 as the main Firefox version for Jaunty Jackalope users and since Firefox 3.1/3.5 was made available as a Beta (3.5b4) at the time, they apparently decided it should keep its beta name (Shiretoko).
full blog post available at http://www.linuxmint.com/blog/
which explains everything in a lot more detail.