Is that the default Firefox browser layout or custom homepage?

Yeah, it is. I mean, that is shown when I hold my cursor on something that has an URL or when browser gets/loads some information. When I move cursor to sites at the homepage the status bar appers to so it was easier to make a screenshot this way. So, it doesn't matter if it is a default Firefox browser layout or custom homepage

Do you only want to hide the overlink message that shows if you hover a link and possibly leave the other messages about loading the page which might be useful in case of issues ?

#statuspanel[type="overLink"] #statuspanel-label { display:none!important; }

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

More info about userContent.css and userChrome.css in case you are not familiar: *https://www.userchrome.org/what-is-userchrome-css.html

• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets = true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

That doesn't help. As I said, there is no Mozilla folder in Roaming folder so there is no Profile folder. (here's a video https://youtu.be/cJbyeAnD07U). Is it because I downloaded the browser from Microsoft Store?

Oh yes, the Mozilla Store profile is buried in an unusual location. I don't know whether it is documented in any formal articles, but it has been mentioned in a few threads. For example:

https://support.mozilla.org/en-US/questions/1394270#answer-1543318