This is what Firefox FAQ. https://support.mozilla.org/en-US/kb/install-older-version-firefox https://support.mozilla.org/en-US/questions/1141234

''<p>jwedge1726 [[#question-1426509|said]]</p>'' <blockquote>I have an older piece of equipment that I need to communicate with it and the 232 interface will only work through Firefox. I was told I can downgrade the current version to 3.0, which is what I need, but I cannot find out how. HELP Jeff </blockquote> Firefox 3.0 is ancient as it was released way back on June 17, 20'''08''' https://www.mozilla.org/firefox/releases/ If this is a Windows system then perhaps your best option is to use the portable Firefox 3.0. The portable Firefox builds are self contained as they can run off hard drive or even completely off a usb flash drive. They will not interfere with any of your current Firefox installs you may have. https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox,%20Portable%20Ed./Mozilla%20Firefox,%20Portable%20Edition%203.0/ The English locale is Firefox_Portable_3.0_en-us.paf.exe

See also: *https://www.google.com/search?sa=N&num=100&q=rs232%20to%20usb