Realise this is old news but felt the urge to comment on it. I have to agree with Michael's and Fleep's comments about Firestorm. I had to move away from Imprudence because I wanted to experiment with mesh. I don't like the SL viewer interface, or Firestorm's, even with the retro interface engaged ( that is all phrased to try and be polite ). But the problems go way beyond that. I for my sins, use Debian Linux of various flavours on a quad core CPU and GTX550 Ti graphics card with the latest proprietary drivers, no slouch of a PC and shouldn't have many problems with modern viewers. I also run my own server with 5 regions attached to OSGrid, which is an advantage when it comes to evaluating these viewers.
The reality is not encouraging. This is my experience in brief. Imprudence is snappy to use even with the graphics set a high as possible, always returned around 30-40 fps mark with the occasional dip to the top teens, rarely crashed, but does have issues with inventories occasionally as well as animations. As I have more than one PC that I can run simultaneously it's possible to log in with an alt at the same time to observe how other people would see what is going on; that can be quite revealing.
Firestorm immediately cause me problems because of a font library that makes it crash on start up. Without going into details this has taken me 6 weeks to find a solution, and then only by chance. Being forced to install an older version of my OS in a spare corner of my hard drive just to be able to import meshes until I could find a proper solution did not make me happy.
The next problem was the dire frame rate, which like all newer viewers is caused by shadows if you have them enabled; but even with them disabled, I am only getting on average 30 fps very rarely higher, with shadows engaged this drops to around 15fps and lower depending on what is going on. Not only is this bad enough, but it will occasionally freeze solid for no apparent reason, leaving me having to resort to the command line to kill it. With Debian wheezy it will freeze the entire Xserver leaving no choice other the the reset button.
Together with the maddeningly infuriating interface ( any of them ), which I know is not Firestorm's fault, and some things just not working correctly, or missing, I know all the development focus has been on SL in the past, but I suspect the ground to catch up now for Opensim may stretch things more than a little, all in all it's a disheartening experience to try and use, and doesn't endear me to it. It's only virtue is that it renders scenes beautifully, so for photography ... the choice is yours. I can see that I personally will be only using it for uploading meshes and keeping an eye on any future improvements that may occur.
Singularity on the other hand felt more like an old friend the first time I fired it up ( with none of the problems I had with Firestorm ). The interface is for all intents and purposes the same as Imprudence. Finding things in familiar places, and unlike Firestorm, not buried under seemingly endless unrelated menus or pop ups and floaters that seem to jump to odd placements on the screen or be duplicated more than once, helps a great deal. Singularity was not without problems, and I found a few bugs which were duly reported. What surprised me was the fast responses from Siana Gearz explaining some of the Opensim server/viewer problems, ( some of which I must admit went over my head, but in a nice way ) , and who even visited my regions on OSGrid to try and isolate some of them, as far as I am concerned this was above and beyond the call of duty and very much appreciated.
With a few tweaks Singularity is now running sweetly with graphics on Ultra and the Full Monty engaged and still returning acceptable frame rates in the mid 20 to mid 30 fps range consistently, only dropping into the mid teens occasionally. I can view meshes OK and do everything I could with Imprudence, in fact if I disable shadows it is Imprudence. It also renders scenes extremely well btw, with a marked visible difference if you run Imprudence alongside it.
You will gather from this that it has become my viewer of choice. It is IMHO a plugin replacment for Imprudence, which is ageing rapidly. Singularity only lacks mesh upload, it is not perfect but it doesn't crash, lockup, or hog my system, and works well at least for me, Firestorm doesn't!