Firefox will not appear in the Market for most phones with incompatible hardware. You can check if your phone is supported here:

https://wiki.mozilla.org/Mobile/Platforms/Android

On some supported devices, a bug prevents Firefox from appearing in the Market. This may be related to the recent Market update. You can go to Settings/Applications and uninstall the Market update, then find and install Firefox.

Or, if you have a supported phone, you can download the app directly from here:

http://ftp.mozilla.org/pub/mozilla.org/mobile/releases/4.0b3/android-r7/multi/

(Note: To download the app directly for an AT&T phone, you will have to search for instructions on "sideloading" the APK file, since AT&T disables the option to install from non-Market sources.)

Like bheisterka..., I cannot download Firefox. I've got a T-Mobile MyTouch; I don't know HTC's model name for the phone, so I don't know if it's on your supported-devices list. I got the same Cannot Download error message that bheisterka... did. My phone is running Android 2.2. I tried using the QR code, I tried going to the Mozilla site. I looked in the file directory and it said the Firefox download is incomplete and cannot be restarted. I went to the http://ftp.mozilla.org/pub/mozilla.org/mobile/releases/4.0b3/android-r7/multi/ site and saw only a file directory. I clicked on one of the files, but nothing happened. I just saw the file fennec-4.0b3.eabi-arm.apk file in the Recently Downloaded list on the "drawer". I clicked on that and it tried to install, but I got the Application Not Installed error. I tried again. Same thing. I rebooted the phone, went to the directory and clicked on the fennec file there. It has a file size of 0, so it was no surprise that that did not install either. I didn't see Firefox in the Market, so that means my phone isn't supported, right? Maybe that's why I cannot install it.

Sorry, the T-Mobile myTouch 3G and myTouch 3G Slide are not supported.

The newer T-Mobile myTouch 4G is compatible with Firefox 4 beta.