If you just want to disable IPv6 in Firefox, bring up Firefox. In the bar where you type URL's, type 'about
:config' ( without quotes ). You will get a long list of things that can be configured.
There is a field called 'Filter' Put your cursor there, and type IPv6 and as you type you will see the listed things below change. Once you see the item called network.dns.disableIPv6 single click that line, it will highlight. Right click the mouse pointer over the Value field. You should see a pop up appear. There is an option, Toggle, click it, it will change from false to true. Close firefox, restart it, you now have IPv6 disabled in firefox only.
You may want to try changing these items also.
network.http.pipelining -> true
network.http.pipelining.maxrequests -> 8
network.http.proxy.pipelining -> true
Same method as above, the maxrequest item is changed with Modify rather than Toggle.
Hope this helps.