Please consider turning on send info to Mozilla from with in Firefox before posting a question. This helps us help you. I have no info from your system that would help see things that cause issues. As I have no information not even browser version or add-ons or video card driver errors it is guessing. If using version 59.01 on Win10 there appears to be an issue that only affects users who have turned off Cleartype Font rendering within their Windows Settings. So a workaround for the moment would either be to turn Cleartype back on or disabling hardware acceleration in Firefox: https://support.mozilla.org/en-US/kb/performance-settings

Please let us know if this solved your issue or if need further assistance.

Please open Settings --> Apps --> Default Apps then click the Browser that is set as default (probably Edge) and a list will come up, select Firefox from the list. This will make it the default. If is not on the list browse for the Firefox.exe file.

Note : Microsoft has been testing in beta that all Windows Software will open on Edge only. No idea if that will hold true to the Spring Creators Update in the next few weeks.

Please let us know if this solved your issue or if need further assistance.