For some reason within the last week my Quicktime browser plugins just stopped working in ALL of my browsers (Safari, Firefox, Shiira). I cannot play QT VR files or .mp3 or .mov files and my browser window is black and says (no picture). On some sites I get a Quicktime logo image with a question mark. I tried re-installing QT 7.1.1 but it didn't resolve the problem. I checked all of the MIME settings in my QT preferences and made sure the media types were checked. Is it possible that when I installed Real Player it might have hijacked my browser plugins? That is the only thing that has changed since I installed it to watch some streaming RMA files. I'm running OSX 10.3.9 on a G4 450 powermac.

You may be right about RealPlayer. However, something like that happened to me too recently without messing with Real. After an update to QT 7.1.1 my Camino browser stopped seing QT (same logo with a question mark). Reinstalling Camino after the update did the trick. Maybe you need to reinstall the browsers or at least get rid of all their preferences in your user account/library directory. Just keep the bookmarks file on the desktop and put it back after browser relaunch.
Hope this helps.
Alex
2.3 GHz DP G5, OS 10.4.6

I tried reinstalling my browsers and I trashed Real Player and Ad Blocker and reinstalled Quicktime. No dice. Firefox just gives me the spinning ball of death and crashes whenever I try to download .mov files. This is really frustrating.

Since the problem affects all those browsers, and
since you've done so much reinstalling... I wonder
if deleting some user files they share would help:

this folder:
~/Library/Caches/QuickTime

and these files:
~/Library/Preferences/QuickTime Preferences
~/Library/Preferences/com.apple.quicktime.plugin.preferences.plist

Move them to the trash.
(note ~ is the path to your Home)

-HI-

Open your HD/Library/Internet Plug-Ins folder.
Drag the one with VLC in its name to your Desktop (so you can put it back if you want).
Quit and relaunch your browser and try the page again.

Removing the VLC plugin and cache folder did the trick! QT7 is now working again in Safari and Firefox (although some inline Flash browser images still appear with the QT logo and question mark in Firefox only but it is specific to that browser). I can now view .mov and QT VR files again in all of my browsers! Thanks so much QT Kirk! I have been racking my brain over this for weeks!

Glad to help. Now let's fix the other issue.
Open the QuickTime System Preferences/Advanced tab and toggle the disclosure triangle next to Miscellaneous. Remove the check mark for Flash Media (.swf).
Download and install Flash Player and its plug-in (Adobe). Restart your browser for these changes to take effect.

I opened my QT preferences and the micellaneous settings were already unchecked so I just reinstalled Flash 8 and all systems are fully functional! Thanks again!

I've tried all of these suggestions and I'm still seeing the QuickTime logo with the question mark. Should I just get rid of QuickTime all together and start over?

Bump.

Having the same issue here re: quicktime and Firefox. Tried the fixes listed here to no avail. Still get the Quicktime Logo w/ question mark.