First get the HWND for the address text box in teh firefox exlporer. you can use programs like Spy++ (I forgot what it was called, but it came with Microsoft Visual Studio 6.0). The you can use SendMessage and send a WM_GETTEXT message to it.
Here is an example
I haven't tested this out yet, so there might be a few errors from the LPSTR to CHAR conversion. I also didn't get the correct names for the firefox menus.
HWND hFireFox, hAddressBox;
hFireFox = FindWindow("FireFox", "FireFox");
hAddressBox = FindWindowEx(hFireFox, NULL, MAKEINTATOM(0x800), "AddressBox"); //Forgot what the 3rd parameter does
SendMessage(hAddressBox, WM_GETTEXT, MAX_PATH, szAddressBox);
To check use strcmp() and use WM_SETTEXT to send text to firefox
if (strcmp(hAddressBox, "www.google.de") != 0)
\\if false then...
SendMessage(hAddressBox, WM_SETTEXT, NULL, "www.google.de\0");