slow scrolling on all pages - Gutsy

Binary package hint: mozilla-firefox

I just upgraded my existing Feisty system to Gutsy in place (sudo do-release-upgrade). Now scrolling in Firefox 2.0.0.6 using the mouse wheel, arrow keys, or pgUp/Dn is very very slow and jerky, and uses up 100% of my CPU cycles for several seconds. The length of time that the CPU is occupied for seems to vary independently of how far I scroll. Grabbing the scroll bar and dragging it up and down seems to be affected by this issue too, but less so, and so it performs acceptably. This occurs on all pages (all pages long enough that I can scroll on them, that is), including locally-stored plain text files. Disabling (but not uninstalling) all extensions has no effect. The only themes I have installed are the default Firefox theme and the three Ubuntu themes. I do have Ubufox installed (but I did disable it, to no avail). My plugin list (gleaned from about:plugins) is:

      File name: libflashplayer.so
      File name: libflashplayer.so
      File name: libtotem-basic-plugin.so
      File name: libtotem-gmp-plugin.so
      File name: libtotem-mully-plugin.so
      File name: libtotem-narrowspace-plugin.so
      File name: libjavaplugin_oji.so
      File name: mplayerplug-in-dvx.so
      File name: mplayerplug-in-qt.so
      File name: mplayerplug-in-rm.so
      File name: mplayerplug-in-wmp.so
      File name: mplayerplug-in.so
      File name: libflash-mozplugin.so

Now that this text box has gotten full enough to have its own scrollbar, I observe the same behavior (using the mouse wheel) when scrolling the text box. I have seen several other bugs which involved this issue only on some webpages, and generally they are older bugs, while this is clearly a new issue. I was running Firefox 2.0.0.6 prior to the Gutsy upgrade with no problem. I have also tried

$ export MOZ_DISABLE_PANGO=1
$ firefox

, as recommended in Bug #74116, but this did not help either.