Firefox is free and there is no real warranty, this warning is merely meant to warn you that you need to be careful with changing prefs on about:config as there are guides all around internet to make changes on about:config like how to speed up Firefox or increase the privacy that can have a serious impact on how Firefox functions. As long as you knows what the prefs you change are about and are aware of possible caveats, you can make simple changes to enable/disable some features. The same with using CSS code in userChrome.css/userContent.css that also can have negative impact and possibly hide or disable features or otherwise cripple the browser, we regularly see question about a broken userChrome.css because of changes made in a next release. The devs have chosen to only include a user interface for important settings that are used by a wider audience, they regularly gather telemetry on how many use a specific feature. This is because each feature needs to be tested for all platforms to make sure it keeps working properly and that it doesn't have a negative impact.

I still have 1 year laptop warranty, will that warranty become void if I did all of those things listed? The firefox itself didn't come within the laptop though (not pre-installed). And what warranty was that user talking about?

Nothing to do with your laptop warranty, just a funny place on words to be careful. The about:config is meant for advanced users as there are some preferences that could make Firefox work in a way you were not expecting, have issues or not be stable etc. It is basically saying "use about:config at your own risk" for your Firefox install.

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I'm sorry if this seems like a silly question. I was totally concerned that messing with about:config might void my laptop's warranty, which still has 1 year left. So, there's really nothing I need to worry about, right?