HTC Desire is below our minimum RAM requirement. We are not confident that it provides an acceptable performance level. If you would like to try it anyways you can get it from http://ftp.mozilla.org/pub/mobile/releases/14.0/android/multi/

On the dutch website the HTC Desire is noted as a supported device. It states that you need a phone with at least 512MB RAM an ARMv7 processor and Android 2.2 installed. Sounds like the HTC Desire matches those all, even the RAM.

http://support.mozilla.org/nl/kb/werkt-firefox-op-mijn-mobiele-apparaat

I have the same problem, after i updated to the official HTC 2.3.3 FW. Direct install works fine. It runs on SDRAM, but much data is stored in memory...