Firefox: Why is it slow once I have some active downloads?

"Unless this also happens with downloads from slow sites, I'd say there is a good chance your download(s) are saturating your bandwidth, causing page loads to struggle to get through the same channel.

Note: if this is the case, basically the only solution is to use some plugin or separate app for downloads that is capable of limiting the bandwidth used by downloads."

Try using a download manager separate from the built in version. That is how I stopped FF from getting wrapped around the axle.