How can I switch tabs in Notepad++ via Ctrl+PgUp/Down?

I'm using Notepad++ version 5.7, and it's simple to change the behavior to do exactly what you're looking for -- use Ctrl+PgUp/PgDown to change tabs just like in Firefox. Here's how:

Step 1: Click Settings | Shortcut Mapper... and make sure "Main menu" is highlighted at the top of the window. Then scroll to the very bottom of the list, and find the line that says "Switch to previous document". Double click on it's current shortcut value, and in the window that comes up change the shortcut to "Ctrl + Page up". Similarly, change the shortcut for "Switch to next document" to "Ctrl + Page down".

Step 2: Click Settings | Preferences... | MISC. tab. In the "Document Switcher (Ctrl+TAB)" box, uncheck "Enable MRU behavior". This is needed so that switching to the next/previous document will switch to an adjacent tab, rather than switching to the Most Recently Used tab.

EDIT: shoot, this question is answered far better here:

Switching to tabs to the right or left of the current tab in Notepad++

Wish I'd seen that first! :)

Use Ctrl + Tab . Same will apply to most tabbed applications. :)