If in your original post, you posted the <exact> error message you see, a lot of speculation and options could have been avoided.
Based on your original error description “no longer have an Internet connection” and “like firefox can’t resolve an address.”
The first generally speaks to fundamental network connectivity, and the second suggests basic networking is working but there might be a name resolution issue. This is where some basic knowledge of network troubleshooting is required, starting with PING and possibly TRACEROUTE.
Once you narrow down the problem to fundamental networking or a service running on top of networking, you can proceed to narrow down the problem further… If a networking problem, then physical connections and basic network configuration. drivers, etc. If a name resolution problem, then inspecting /etc/resolve.conf and using the nslookup utility.
You mentioned using Firefox, part of your early troubleshooting should have been to determine if your network connectivity problems were Firefox-specific or affects multiple or other apps.
With a lot of guessing, you also mentioned a possible proxy setting issue… But the way you described and didn’t post the exact message, it’s unknown if it was simply a warning or something critical to your connectivity.
If you do have some kind of proxy setting issue (and running a local proxy on localhost or 127.0.0.1 is common),
You should know that your system can have system proxy settings defined a number of different ways, typically in YAST/Wicked (Network Adapter settings), even some other way (command line? Locally installed proxy app?).
Web Browsers also contain their own application-specific proxy settings (would affect only that web browser and no other app), which can typically found in the general area… In FF,
Preferences > Advanced Settings > Network > Settings
FF is somewhat unique in that the default setting is “Use system settings” which for most browsers would be the same as “No Proxy” (By specifying no Web Browser proxy, the browser would normally still be subject to system settings). The default settings for most browsers is to automatically detect settings which might be provided over the network(eg WPAD). You can set this however you wish if you don’t want FF to use a proxy or even try to use a proxy.
So, unless you think you have resolved your problem already, I recommend you start this thread over with posting the <exact> errors you’re seeing. Don’t paraphrase, the exact wording is often associated with a specific problem.