Why is Firefox so slow (and degrades over time)?

[joe at actionline.com]

My system has all the latest updates for Linux Mint including the latest
available Firefox 27.0.1 for Linux Mint.

However, for quite some time, I have had ongoing problems with Firefox
being very slow ... too often, frustratingly slow. It usually takes about
15 seconds for the Firefox browser to start.  But once started, I can
open an additional session in about 5 seconds.

And, the longer I run my system without hard rebooting, the slower
Firefox gets. Even to shut down KDE and restart it doesn't seem to help.

Chrome, on the other hand, always boots up in less than 2 seconds.

Also, from time to time, I get a pop-up that tells me to "Close Firefox"
( see this link: http://www.upquick.com/temp/ff1.jpg ) yet there is no
indication of any Firefox functions running.  So, hard reboot again.

What can I do to speed up Firefox?