Within minutes of launching Firefox it becomes almost unusable. System fans start running and overall performance of the mac book pro takes a huge hit. MacBook Pro (Retina, Mid 2012) Processor 2.3 GHz Intel Core i7 Memory 16 GB 1600 MHz DDR3 Graphics NVIDIA GeForce GT 650M 1024 MB Intel HD Graphics 4000 1536 MB 1 display setup Built-in 15.4-inch resolution setting (2880 x 1800) 3 display setup Built-in 15.4-inch resolution setting (2880 x 1800) Dual Samsung U28E510 Displays 28-inch resolution setting(1920 x 1080) 28-inch resolution setting(3840 x 2160) Have completely removed Firefox and reinstalled from fresh Firefox 59.0.2.dmg, No Change in issue. (No add-ons outside of default install) Started in Safe mode, No Change in issue. Disabled Use recommended performance settings, No Change in issue. Adjusted Content process limit above and below the default of 4 , No Change in issue. Disabled Use hardware acceleration when available, No Change in issue. Disabled accessibility services from using your browser, No Change in issue. Single Built-in Display only or 3 display mode, No Change in issue. Safari Version 11.1 (13605.1.33.1.2) Works Correctly. Google Chrome Version 66.0.3359.117 (Official Build) (64-bit) Works Correctly.