First off How Much RAM does this system have? 59% might be normal if 8GB or less RAM.
I have noticed with Firefox that the longer the browser is open the larger the RAM consumption is... I personally close Firefox when done with it and relaunch the browser when I need it again. Systems with 8GB or more RAM while Firefox can grow to consuming up to around 4GB RAM there is little impact on the user experience as long as the CPU is decent to handle the memory juggle as the browser bloats caching data.
Chrome browser does far better at memory management as well as if its an older computer Chrome will run smoother than Firefox for example I recently installed Windows 10 64-bit to an Athlon 64 x2 1.9Ghz 3600+ CPU system with 6.5GB DDR2 RAM ( 3 x 2GB sticks + 512MB stick ) and Radeon HD5450 1GB Video Card and Firefox has a stutter to the slider bar on all web pages, yet Chrome runs smooth when using the slider bar to scroll up and down on a pages such as facebook.