When first learning keyboard shortcuts, I typically look for a English relationship between the letter of the shortcut and the function I need to accomplish. But for some shortcuts, the letter chosen seems to have no connection with a word that's related to what you're trying to do. That's because there are really two types of letter-based shortcuts: 1) shortcuts chosen for their connection with a word, and 2) shortcuts chosen for their location on the keyboard.
For example, in Windows we're used to common shortcuts like Ctrl+S for Save, Ctrl+B for Bold, Ctrl+I for Italic, and so on. But we're also used to things like Ctrl+Z for Undo, Ctrl+X for Cut, and Ctrl+V for Paste. Why didn't they choose Ctrl+U for Undo? Because Ctrl+Z is easier to press, and Undo is a function we use a lot. The same is true for Ctrl+V: 'V' has nothing to do with the word "Paste," but the function of pasting is so related to the function of copying, it's nice to have the copy and paste shortcut keys next to each other. Actually, the keyboard shortcut letters for Cut, Copy, and Paste (X, C, V) seem to have been chosen primarily for their location on the keyboard. The fact that 'C' matches Copy and 'X' has a linguistic association with Cut is just an added bonus.
FireFox also includes a healthy mix of word-based and location-based Ctrl-key shortcuts. For example, Ctrl+T for a new Tab, Ctrl+P to Print, Ctrl+R to Reload, etc. But there are also many other keyboard shortcuts that have been chosen strictly for their location on the keyboard. Take a look:
Most of these letters have nothing to do with their associated action, but they are very comfortable to use. Notice the grouping of Ctrl+L and Ctrl+K. Like the shortcuts for Cut, Copy, and Paste, getting into the Location Bar and getting into the web search box are common functions and go well together. The addition of Ctrl+J hanging off to the left to open the list of downloads is also appropriate, since it's also a function that involves navigation (though on the local file system).
Once I discovered the reasoning behind why certain FireFox shortcut keys were chosen, it helped me to start using them more frequently. Location-based shortcut keys aren't as obvious, but they are a pleasure to use.
A well-designed application will choose its keyboard shortcuts based on location and ease-of-use while at the same time choosing obvious language-based associations when possible to aid in memorization. Save is a common function, and Ctrl+S is easy to press, so choosing S as the letter of the shortcut makes sense. But when you're setting up a shortcut for a commonly used application function and the most obvious letter is already taken or is not easy to reach, pick one that's easy to press and forget about the letter.