It depends on how old your flashcart is. If it used to work on the 3DS, but was blocked by a firmware update, you can use flashcarttimewarp to make it work again. Flashcarttimewarp can be found on "some iso site." If it never worked on the 3DS, there is no way to make it work on the 3DS.
Try using the Homebrew Launcher to launch EmuNAND9 instead of the browser.
There's not much difference between 9.0 and 9.2. Everything for 9.2 works for 9.0 as well.
If you wanted to update to 9.2, you would need to use SysUpdater and Pasta CFW or rxTools' devmode/pasta mode. Again, there's not much point in doing this since you're on 9.0.
Pasta CFW is a very limited CFW. It does not support emuNAND and does not use firmlaunch to spoof the 3DS's firmware version.
Cake I don't have any experience with. I believe it is a more lightweight CFW which supports emuNAND and firmlaunch.
RxTools is a CFW with a ton of features. It has FBI injection, xorpad dumping, game decryption, options for launching with or without firmlaunch and a few more features. It also supports emuNAND.
RxTools is generally considered the best CFW for the O3DS.