I can not help wondering is it worth using an AV program or suite that can not stay compatible with your browser! If you have an official version of XP presumably a free Windows AV program is available.

What is the AV system in use ?
Check with their support site/forum it may now be updated to be compatible with Firefox.

We do not normally recommend using old versions of Firefox, they have security problems. When a new version is released security fixes are included and announced, these then become problems likely to be exploited. You will put at risk your personal data and computer security if you use an old version.

(Unless extreme precautions are taken, such as using on private networks behind sophisticated firewalls, putting it on a Virtual Machine, or having a well rehearsed backup and bare metal restore policy)

you can get version 12 here

https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/12.0/win32/en-GB/

Regarding the above link, if you prefer another form of English, or another language altogether, navigate up a level so you can select the best download for your needs.

Thanks.