Until yesterday I was able to open it, but now I've tried both my 3DS and 4.5 says that I have to update the firmware to use Browser
So you're saying you were able to open the browser on 4.5 yesterday, but not today? What is your exact firmware on this 4.5 3DS? Please go into system settings and get the full version (4.5.0-?? <- We need to know what the last two numbers before the "E" are).