Many have reported the same issue. See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Hi nennir11, Firefox 49 has an incompatibility with some older graphics card/chipset driver software. Some users have reported that switching between tabs will cause Firefox to fix the display, but that's obviously not convenient to do all the time. A bug report is on file ([https://bugzilla.mozilla.org/show_bug.cgi?id=1304360 bug 1304360]). You can help the developers working on the patch by providing some information about your current Graphics software (see below). Then try to fix it by updating your graphics software, or if necessary, work around it by turning off Firefox's use of hardware acceleration. Here are the detailed steps: '''''Your Current Graphics Software''''' You can copy/paste this info from the Troubleshooting Information page. There are 3 different options for opening that: * "3-bar" menu button > "?" button > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste about:support in the address bar and press Enter On the left side, click the Copy Text to Clipboard button. Then paste into a new reply here: https://support.mozilla.org/questions/1141267#question-reply Before posting, please skim down and remove any lines that contain your email address or any other sensitive information (usually there isn't any, but just in case). You also can delete all the lines about printers if you like. '''''Updating Your Graphics Software''''' Fred linked you to our support article suggesting safe ways to check whether graphics card/chipset driver updates are available for your system. Unfortunately for some older systems, updates are just not available. '''''Working Around Incompatible Graphics Software''''' '''Option #1: Targeted Change''' This should have the least performance impact, if it works: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''d3d''' and pause while the list is filtered (3) Double-click the '''layers.allow-d3d9-fallback''' preference to switch the value from true to false You might not notice any change until the next time you exit Firefox and start it up again. '''Option #2: Disable Hardware Acceleration''' This is a common way to bypass most graphics software incompatibilities, but may cause fonts to appear spindly and poor animation performance. "3-bar" menu button (or Tools menu) > Options In the left column, click Advanced. On the right side, with the "General" mini-tab active, uncheck the box for "Use hardware acceleration when available". This takes effect the next time you exit Firefox and start it up again.