My primary method of getting through big folders is by viewing in the thread pane, highlighting groups of messages with combinations of shift/control click, and mass-deleting them (or, in the news case, marking them read). In mozilla, control-click works, but shift-click (much more important to me since it's the only way to select a range) does not. I'm not sure if this is a tree widget bug or a mail FE bug.
*** This bug has been marked as a duplicate of 12288 ***
Mark it a verified/dup.