It is 9.0.1-1 (mepis)
OK Rob what you need is this version; "firefox_9.0.1+build1-0mcr85+1_amd64"
You can get that by enabling the Community Repository "cr" repository for Mepis-8.5 in synaptic.
Add the below to synaptic's: "Settings > Repositories"
Save the entry and then hit "Reload". Then using the quick search, type firefox into the box and hit return or click the binocular icon. You'll then see that there is more than one version of 'firefox' available under the "Versions" tab in the "details" section.[EDIT] I can't remember offhand whether I had to set "Preferences> Distribution" to prefer Mepis rather than 'highest version' If firefox version I detailed doesn't show up then you'll have to change that setting.
Remove your current version first then return to the same point and install the version detailed above.
Wow I hope that all makes sense when read... GL