HERES A WORKAROUND I just discovered (at least, it works in FireFox- haven't got around to trying it in other browsers)
When the window freezes, simply "Duplicate" that tab (Right-Click on the relevant (white coloured, not blue) Tab at the top, and choose "Duplicate"
The new tab will have all
your text, not just what's currently visible, and it won't be frozen.
- - - -
this problem may be related to the software
for Intellimouse? - haven't played around enough to find out.
In my case, occasionally the cursor arrow switches (apparently spontaneously) to the "solid triangle with a dot" which indicates autoscrolling, and from that moment forth, no further typing (and usually, no scrolling) can be done
Which until now means I can only grab a screenshot of what's showing in the message window (the one we use to compose or edit a message)
and close that browser tab, open a new one, and re-type my message from scratch.