There is a workaround for this... when Firefox asks you to select a program to open with, click on save file and hit ok. In the field "save as" enter the name of the statement with a pdf extension at the end. (i.e. August2011.pdf) this will create a pdf of your bank statement, which can then be opened with Preview.

It's not the cleanest solution. If anyone knows of a better solution like a plug-in or something please reply to my post. Thank you.