When toolbar buttons are displayed in text only (i.e., no icons), the Greasemonkey button has only the word 'Greasemonkey' both when it's enabled and when it's disabled. It would be useful to have different button legends so that it would not be necessary to click on the arrow in order to find out whether Greasemonkey is enabled or disabled.
We could use something like 'Greasemonkey on' and 'Greasemonkey off', but some people might draw opposite inferences from them (because some people might view the text as status, while others may view the text as a command).
I recommend leaving the ON state as it now is and changing the OFF state text to the same word, but with strike-through text. If strikethrough text is not possible, I recommend using 'Greasemonkey OFF' and 'Greasemonkey ON'.
Colors might help some people, too, but, being partially color-blind, I would not recommend that as the only differentiation between the two states.
Strikethrough on disable is probably a ~1 line CSS change.
Strike through the text of the disabled toolbar button.