Weird Firefox bug or feature (?)

When I hold down CTRL SHIFT and double click on text or an image it disappears while using Firefox 1.5 with the following extensions:

DOM Inspector Talkback Numbered Tabs Adblock ColorZilla IE View MeasureIt Tabbrowser Preferences Web Developer SearchStatus Forecastfox ConQuery undoclosetab SpellBound Mozilla SpellCheck Libraries (for SpellBound) Duplicate Tab JavaScript Debugger PDF Download

A bug? A feature of one of the extensions? A bug of one of the extensions? I don’t know.