For future planning put aside a few currency units ($, £, €) a week and you'll be well placed to go trawling for a 5 year old iMac on eBay or such every 3 or 4 years.
Changing the browser user-agent string to identify as the latest version of Firefox may work (no guarantee) using something like this add-on:
https://addons.mozilla.org/en-US/fir...tcher-revived/
Change the Firefox on MacOS string to the latest from here:
http://www.whatismybrowser.com/guide...-agent/firefox
Test to see if the setting change has taken here:
https://www.whatismybrowser.com/dete...my-user-agent/