If you learn to code a little in Python ,can you make changes to things like the menu bars and fonts in Mojave?Is stuff like that available to
developers or does Apple make it impossible to access stuff like that?
Most of what you want to do is in the system area of the OS and Apple has increasingly blocked user access to that area for security. To compensate, they do make some customization available through the System Preferences. You've already been given the "how to" of making the fonts and menu bars larger in other threads where you have made the same kind of request. If the options there are not sufficient, you can search for third party tools to modify Mojave through whatever search engine you may use (Google, DuckDuckGo, Bing, whatever). Beware of those third party tools, as anything powerful enough to get into the secure area will be powerful enough to wreak havoc in your system.
If Firefox allows you to change the font, you can. Apps don't give you arbitrary access to change things within them. Text editing apps, where you enter a lot of arbitrary text, give you access to all the fonts on the system to be applied to the text you enter. But they don't allow you to apply that font to their menu options or other content.
Swift is a language developed by Apple. You download Xcode, a free development IDE, from the Mac App Store and can develop iOS/watchOS/macOS/atvOS apps using either ObjC or Swift.
Stanford has a nice CS class that deals with iOS development with Swift that you might want to follow along to see what is entailed and how hard or trivial programming is. The entire class video is posted online.