Type about:config in the address line to go "behind the scenes" in Firefox.
Be careful so you don't accidentally change other settings.
To reduce memory usage:
Set browser.sessionhistory.max_total_viewers to 0.
To increase browsing speed:
Set network.http.pipelining to true.
Set network.http.proxy.pipelining to true.
Set network.http.pipelining.maxrequests to 10.
Then right click --> New --> Integer, to add a new line and name it nglayout.initialpaint.delay and set the value to 0.
To reduce memory usage when Firefox is minimized:
Right click --> New --> Boolean, to add a new line and name it config.trim_on_minimize and set the value to true.
This will make Firefox noticeably faster.