That file was probably created by Linpus or by Acer specifically for their program repository and not available from other sources, Mozilla only offers tar packages for Linux.
https://support.mozilla.com/en-US/kb/Installing+Firefox+on+Linux

That version of Linpus Lite may not even have the correct "packages" for Firefox 3+ versions, the key "package" is usually the GTK package being too old.
http://www.mozilla.com/en-US/firefox/system-requirements-v3.html
http://www.mozilla.com/en-US/firefox/system-requirements.html - 3.5 & 3.6

Linux on Netbooks only seemed to be popular for a short time here in the US, and IMO, Microsoft killed that "wave" by extending the date they allowed OEM's to sell WinXP Netbooks - until MS could come up with a Win7 Netbook OS. (That "date" was almost 2 years later than they allowed the OEM's to install WinXP on any other type of PC. The cost to OEM's was in the range of a couple of dollars, almost matching the zero-cost of Linux to consumers. "Smart marketing" or taking advantage of their "leverage" over OEM's and MS's almost monopolistic control of the OS market to OEM's?) As a result, there's little incentive for the various Netbook maker's to keep their Linux software repositories updated and to keep developing the operating system to keep pace with other Linux distros. MS shut that down fast, IMO.
http://distrowatch.com/

I have an ASUS EeePC 900A, which I bought in Aug 2008, that came with Xandros Linux - Newegg offered me a WinXP OEM disk for $10 with the purchase of the EeePC. That version of Xandros Linux wasn't ever updated by ASUS to be able to run Firefox 3, but I found some Linux forum threads that provided a sh script that would both update the GTK package that Xandros needed for Firefox 3+ versions and install Firefox 3. After the update / installation was complete, I discovered how slow Firefox 3 ran on that old Xandros Linux operating system - it was worse with Firefox 3 than my old Pentium II 350MHz PC w/512 MB RAM was, even though the EeePC has a 900MHz processor and double the RAM (1GB). I dumped Xandros and installed the Ubuntu derived EasyPeasy OS, that came with Firefox 3.6 .

My advice is to install something like the Ubuntu Netbook Edition operating system, or one of the Ubuntu-offshoots like EasyPeasy; both of which come with one of the latest versions of Firefox, and are active projects.

http://www.geteasypeasy.com/
http://www.ubuntu.com/netbook

Thanks for the reply

I've looked at EasyPeasy and might give it a go but I'm a bit concerned that my 3g mobile dongle might not work. Don't have a problem with it on linpus tho.

Thanks again Tom

You might want to check about that 3G dongle in the EasyPeasy forum or at the Ubuntu forums for user feedback.
http://forums.geteasypeasy.com/
http://ubuntuforums.org/

I would expect better support for hardware like that from an Ubuntu off-shoot like EasyPeasy, than I would from an older OS like Linpus.

I put Ubuntu netbook remix on a while ago and the dongle worked fine but the system wasn't as responsive as with Linpus on. That's why I was after the update from acer but if it's no longer available I might have to put up with some lag or try EasyPeasy

Cheers Tom