Firefox shouldn’t feel slow, particularly when compared to Opera and Internet Explorer. But sometimes Firefox can become slow. This happened to my browser that had been in active use for years.
When I started to search for solutions, found this: 5 Things To Do When Firefox Runs Slow But Other Browsers Run Fast article gives some tips to try to restore Firefox it to its full glory. I tries some of the tips to see if you can restore it to its full glory. The tips I tried really helped. The tips that helped me best were SpeedyFox and looking for extensions that slow down the browser.
The other task was finding the extensions that slow down the browser. For that both 5 Things To Do When Firefox Runs Slow But Other Browsers Run Fast and Browser Slow? How to Make Mozilla Firefox Fast Again articles gave useful tips.
Also Mozilla documentation chapters Firefox Health Report – understand your browser performance and Firefox Health Report says your browser is running slower than usual chapers were somewhat helpful.
Last important step was to makes sure that I had updated to the latest Firefox version. I downloaded it from Mozilla home page. Also installing Flashblock to block annoying and slowng down Flash content helped also (my security software from F-secure seems to slow down loading of pages with Flash content considerably).