Pages 104 and 105 of the Canon EOS 50D Instruction Manual (click to download the PDF) describes how to prep the camera for shooting BULB exposures.

Three main steps are involved:

1. Set the Mode Dial to M.
2. Set the shutter speed to "buLb".
3. Set the desired aperture and shoot.

This is a screen capture of Page 104:

Page 105 touches on the use of the Remote Switch RS-80N3 or Time Remote Controller TC-80N3, or any other EOS accessory equipped with an N3-type terminal (see Remote Shutter Release / Trigger Options for Mid to High End Canon Digital SLR Cameras for examples) for shooting BULB photos.

I found two posts on DPReview that had some recommendations for remote control units that could be used to trigger the Canon EOS 50D when shooting fireworks.

idigi wrote:

I use Phottix Mega Cleon on my 50D.

1) When you press the button on the wireless remote halfway – it initiates autofocus.

2) On the wired remote you can switch to the 'bulb' mode that will hold the shutter open until you move the switch back. You don't have to hold the button down. However, it does not have the timer.

I shot some fireworks a week ago with Phottix in 'bulb' mode. I used different timing to get different effects for fireworks. I was able to preview the image quickly on a screen, which helped me to decide on the timing for the next one. I set the lens to manual focus.

chevysales wrote that the best remote for the 50D was …

phottix on ebay…best deal out there… wired or wireless.

i love mine worked great with fireworks.

You might like to refer to the following posts on DPNotes that talk about the Phottix remote units:

• Phottix Cleon wireless / wired remote for the Canon 5D Mark II, 50D, Digital Rebel XSi and other Canon digital SLR cameras.
• Remote Shutter Release / Trigger Options for Mid to High End Canon Digital SLR Cameras.
• Phottix Plato Review.
• Phottix Nikos Review.