MacOS – Lion: Change Keyboard shortcut CMD + ALT + Left/Right Arrow for spaces OR Firefox’s next/previous tab

The built in methods for changing shortcuts are less limited than you think.

Go to the Exposé & Spaces preference pane in System Preferences, and check out the bottom section of the preference pane. Note especially the text just above it:

Keyboard and Mouse Shortcuts (for additional choices press Shift, Control, Option, or Command)

Here's me pressing ⌃⇧⌘ (Control + Shift + Command) while the shortcut drop-down is active:

The shortcuts have the keys I'm pressing added to them (well, sort of, it avoids redundant combinations but its very easy to see how it works and choose exactly what you want).

So for ctrl+alt+arrow keys, open the drop-down, press ⌃⌥ (Control + Option) and you'll note the top item becomes the shortcut you're after.

You can also do the same for the shortcuts to switch directly to numbered spaces or for the Spaces activation shortcut.

Go to System Preferences -> Keyboard -> Keyboard Shortcuts. Then, on the left side, select Exposé & Spaces. On the right side you will see the current keyboard shortcuts for Exposé and Spaces. If you double click on the shortcut command (literally double click ^1 for instance) you will be able to change this to whatever you want. Just do the key-combo you want to use.

So, once you double-click on the shortcut, it'll show it as editable. Then, press the keys you want the short cut to be, all at once, for instance press Apple and J at the same time, and it'll change the shortcut.