I am running Firefox 23.0 on Ubuntu 13.04 (64-bit) and it is a real pain at the best of times.
It takes around 45 seconds to have a usable window after I click on Firefox in the launcher: 15-20 seconds for a window to show, and then 25-30 seconds of a blank window before it is usable.
Many web-pages that I use regularly cause Firefox to freeze up for around 10-20 seconds before it is usable again.
I first thought there was just a bug in Firefox 21/22, so I waited a few versions before speaking up. I installed a clean Ubuntu 13.04 a month or two ago.
In comparison, LibreOffice and Gimp are both usable within 10 seconds after I click on them. The problem does not, therefore, seem to be hardware-related.
I would appreciate any help! Just please don't tell me to switch to Chrome/ium. Firefox worked perfectly on Ubuntu 12.10 before I clean-installed Ubuntu 13.04.