I have increased the font size by going to System Settings -> Font. This changed it for some applications like Konsole and File explorer.
But this doesn't reflect in other applications like Firefox and Intellij.
The "System Settings" and "Font" dialog will (as a rule) affect only desktop applications which have been integrated with it. In your configuration, those are KDE applications.
In X, most applications manage their fontsizes by themselves, and are not affected by these dialogs. Or, even if there is some nominal integration, it may not be maintained. For instance, the thread firefox ignores system font settings on ubuntu summarizes the cause by
The problem is that (non-repository) Firefox builds in Ubuntu or any Linux distro, by default, uses it's own Cairo rather than system Cairo for rendering UI fonts. I forgot which bug report (Bugzilla, Launchpad) I found this in since it was some time ago but the fix is fairly easy.
along with a fix for that application:
In about.config set