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Firefox and Thunderbird have a quite annoying feature (enabled by default): they select not only the words that you double-click, but the space behind them too. You can turn this off by setting the layout.word_select.eat_space_to_next_word to false.

In Firefox you can do this by typing about:config in the address bar, then scroll to the setting with the name from above and double-click it (this will change the default "true" to "false").

In Thunderbird you will have to either edit the user_prefs.js config file directly or install the AboutConfig extension to get the same config editor like the one already present in Firefox by default.