If you had Auto update enabled (which is enable by default anyway) then the next time you start Firefox it should download and install the newest version of Firefox for you.
To find if Auto update is enabled,
1. Open Firefox, click Tools – Options – Advanced
2. Click Update tab and see if
“Firefox” is ticked under Automatically check for updates and
“Automatically download and install the update” under When updates are found for Firefox.
Also, you may click Help – Check for Updates menu item as well to download and install Firefox 3.5.4 or download directly from here
The following are the list of Security issues order from Crtical to Low priority:
Crashes with evidence of memory corruption (rv:22.214.171.124/ 126.96.36.199)
Upgrade media libraries to fix memory safety bugs
Heap buffer overflow in string to number conversion
Chrome privilege escalation in XPCVariant::VariantDataToJS()
Heap buffer overflow in GIF color map parser
Crash with recursive web-worker calls