How to debug Greasemonkey script with the Firebug extension?

Note: this answer refers to old versions of Firefox. Firebug is no longer available, but lives on in the Developer Edition of Firefox.

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Current Firefox and Firebug can now debug current Greasemonkey scripts just like any other javascript. Just find your *.user.js script in the dropdown menu. The console also works.

This works at least on Firefox 28.0 and Firebug 1.12.7; I haven't tried earlier versions.

Note: In order to get it to work, you probably have to set extensions.firebug.filterSystemURLs to false. See "Profiling Greasemonkey scripts" in the Firebug, bug tracker. (Thanks to Shuman)

Updatier: The Mene+Shuman fix now is busted with Firefox 30 and Firebug 2. Firefox 31 may provide workarounds (will investigate). In the meantime, use the "General workaround strategies" listed below.

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Update: This answer is now obsolete.

If you open about:config and
set extensions.firebug.filterSystemURLs to false
then you can use Firebug to debug the Greasemonkey script just like any other.

This works irregardless of the @grant mode.

See Mene's answer -- with an assist from Shuman.

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Old answer:

Because Greasemonkey operates in a sandbox, Firebug cannot see it. There is no easy way around this.

General workaround strategies:

1. Test all parts of a GM script that don't use GM_ functions, in Firebug's JavaScript console first. Minimize use of GM_ functions and don't use GM_log() at all.

2. All of Firebug's console functions work great from within a GM script.