Sudden performance change

I bought a Linux box from PogoLinux about three years ago.
It came with Mozilla as the web access program. I have had
no problems with Mozilla, until about 2 months ago. Very
suddenly, certain previously very quick (<= 1 sec) operations
began to take 10-20 seconds. These operations include killing
a browswer window, and typing or deleting text in the "From:"
box. Creating a new browser window, and deleting a tab within
a window, are still as quick as ever. Moreover, actual network
access performance, e.g., following a link, is unchanged.

Can anyone help with this?

Tom Dailey