If removing add-ons and turning off the firewall have not helped, you can manually remove the network plist files, as this can help resolve wifi problems on several OSX versions and can be particularly effective for macs that have had their OS version updated.

1: Turn off wifi from the Wireless menu.
2: Open Finder, hit [Cmd+Shift+G] and enter the following path: /Library/Preferences/SystemConfiguration/.
3: Within this folder locate and select the following files:

• com.apple.airport.preferences.plist
• com.apple.network.identification.plist
• com.apple.wifi.message-tracer.plist
• NetworkInterfaces.plist
• preferences.plist

4: Move all of these files into a folder on your Desktop called ‘wifi backups’ or something similar – we’re backing these up just in case any unintended changes occur.
5: Reboot your Mac.
6: Turn ON wifi from the wireless network menu again.

This will force OSX to recreate your network configuration files. This may help resolve issues with corrupt or damaged network preferences causing your connection trouble.

The next step to try is to perform an SMC reset and to refresh your NVRAM. As the procedure to perform this will be different depending on the model of your Mac, you can find the steps specific to your machine on Apple's website here: https://support.apple.com/en-us/HT201295.