My apologies on the site being slow for you. I am not sure what is causing that.
Now, I use Firefox as well, and I usually leave it open with at least 3 tabs open consistently, and I do know that this does cause the loading web pages to slow down, at least for me, to the point where I need to reboot my computer because I think something stays resident in memory from Firefox when you simply close and reopen it. I do know that after rebooting, every thing loads very very quickly again.
And there could always be router issues somewhere between your location and the server. I have seen that before, where a router at some intermediary point between me and the server I was attempting to connect to starts going bad, and any traffic going trough it slows way way down because the packets are getting garbled and thus the server has to resend them, which increases load times for web pages
And at the bottom of the forum pages, there is a line that says something like this:
Page created in 0.399 seconds with 10 queries.
which basically tells how long it took the server to respond to the page request. To me, that says any slowness involved has to be occurring during either transmission of the page or in during the rendering process.