You can get the current English (US) Windows release version of Firefox 3.5 from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.5.18/win32/en-US/

It appears that the link supplied by TonyE is no longer valid. I got an HTTP 404 Not Found error. When I shortened the URL to http://releases.mozilla.org/pub/mozilla.org/firefox/releases/ , I saw that there is a version 3.5.19 listed. After thinking about it, I determined that I needed to drill down based upon operating system and language abbreviation. I am still not sure if win32 is acceptable for Windows XP.

I am confused about how to load version 3.5.19. I tried opening the file and downloading the file for Win32/en-US. The name of the file that was downloaded is firefox-3.5.19.complete, but the file type is Microsoft Office Access Report Shortcut and I cannot fine anything with the pages of that file that look like they would help in loading the software. Opening the file instead of downloading it, produced the same result.

Right directory, but apparently Mozilla doesn't support directory browsing with their web server so you have to have the exact URL, http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.5.19/win32/en-US/Firefox%20Setup%203.5.19.exe , or go farther up the directory tree and browse to http://releases.mozilla.org/pub/mozilla.org/firefox/releases/ but without the accidentally included comma that was included earlier in this thread.