Fiddler is a great tool.
However something that drives me crazy is the fact that they have several options on the toolbar, but the most important one, the 'Capture Traffic' toggle is only available from the menu.
I know that you can press F12. However when I am in the middle of development, I can't remember if I turned it back on, or its still off. There is nothing to show you if the 'Capture Traffic' toggle is on or off, without looking at the menu.
I have tried looking online, and here at SO for anything on how to modify the toolbar to add this feature, but I did not find anything.
This is a minor problem, but if someone knows how to add this to the Fiddler toolbar, that would be great.
I'm not sure which version of Fiddler you are using, but in v4.4.5.1 there is a useful button in the status bar:
You can click on it to toggle capturing on or off (and if it is not visible then Fiddler is not capturing:)).