I found this very elegant trick from user ryanker over on macosx.com http://bit.ly/9MjfWN:
If you have your inbox open and wish to select all the emails listed on the page for an action (like delete), you may find that the option of clicking the first , while holding down shift and clicking the last email in a range, to select all intervening entries, will not work if you are using firefox/chrome on a mac.
If the ability to consecutively select is not available, no matter what combination of OS or browser you are using, just copy
into the browser address bar and hit enter. This will select all the messages. Then hit delete/move, whatever.
I was tying to clean out a few hundred emails spanning a year and clicking each one individually was not going to be the answer, till I found this.
Old school, very elegant.