You know when you double click a word this word will get highlighted, but also the space after this word. If you like me find this annoying, this is how you fix it:
1. Type about:config in your address bar
2. Look up layout.word_select.eat_space_to_next_word
3. Double click it to change the value to false
Now you can also highlight two words at once simply by double clicking the space between them (or right after the first word).