firefox grows, taking over all resources

[D. Hugh Redelmeier]

My FIrefox is exhibiting unpleasant behaviour and I wonder if anyone knows 
how to deal with this.

I started Firefox at 1:10 AM this morning and I used it for an hour or
two.  It was OK.  I could browse normally.  Which, for me, means having
lots of windows and tabs open, effectively for weeks.

Now, when I try to continue using it ~7 hours later, it is
non-responsive.  Here's output from top:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
 2502 hugh      20   0 9269m 4.6g   9m R 100.0 79.1 497:19.60 firefox           

It is taking all the CPU available, and has for some time, even while
I sleep.  It is also taking 9G of RAM, 4.6G of it being "resident"
(real, not virtual) on a machine with 6G.

Some web page I have open must be the root of this problem.  As a
further support to this theory, I've had to restart my browser a few
times recently for similar behaviour.  I always open with the same
tabs as before the shutdown.

- is there any way to identify which web pages are the problem?  I
  don't consider a trial-and-error search to be a useful approach.

- is there any way to tell Firefox to limit the resources that it will
  consume on behalf of one web page?

Note: Flash is blamed for all sorts of browser problems.  Not in this
case: I don't have Flash on my system.

I capture the stderr and stdout from Firefox.  This run generated no
messages.
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