@wanliqun wrote the add-on can be used without any patches. I found wrong file paths and line number when using FireLess with Less v2. I also found this errors can be fixed easily, more information can be found at: http://bassjobsen.weblogs.fm/fireless-less-v2/
Just downloaded it and it is actually working. WOW No patch necessary. Only installed the Add On I thought I would have to give up on Less if I could not use firebug. This Add On saved the day. :) Thank you Simone
FireLess is by all means a cutting-edge extension. This very Add-on will make a dent in everyone's workload. Brilliant feature to develop dynamic stylesheet language based Rich Internet Applications and Content Management Systems.