How do I switch the font back to previous in El Capitan? The font changed in El Capitan and, consequently, in some of my programs, the font is off of the screen or cut off. How do I change the font back to the previous font, before El Capitan? 

Examples:  

Eclipse Project Properties Dialog


Eclipse Find Dialog


Firefox Context Menu
You can try installing a third party application called TinkerTool.

TinkerTool is an application that gives you access to additional preference settings Apple has built into OS X. This allows to activate hidden features in the operating system and in some of the applications delivered with the system. You can download the product free of charge.

You will find the new available options in System Preferences and thus you can change the font in the Font tab as in the figure below: