I had a strange case also where I wasn't able to browse with the stock web browser or stock social feeds (Twitter, Facebook). Only Opera and Firefox worked which was really weird. It happened after I tried connecting to a hotspot I made on another device. All my other devices connected to hotspot just fine except for the N9, so I thought it was a compatibility issue until I thought about it more. Maybe it was because I took the SIM card out while the N9 was still on that triggered the problem, I'm guessing. Today, I went to Restore the settings in Reset, and got everything working as should again. Although I lost all my saved Wi-Fi Internet connections. My system theme and icons restored to the default black.

Anyway, I see that your issue is a different one, tioubxancur. Basic task, but I assumed you tried deleting the Wi-Fi connection and reconnecting it. If that doesn't work, you can try turning off the N9 and taking the SIM card out, turn on and see if it connects to any of the Wi-Fi. Restore the settings without SIM card in if you have to. There's also the RestartWifi app that just restarts the Wi-Fi connection when browser's not working, so you don't have to reboot the device. There's probably a tweak out there that lets you choose which connection it should always connect to (like on N900 and N800/810) but I haven't come upon it. The last resort is to flash the device. Otherwise it may be a hardware problem.