One of your add-ons might be the culprit. Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode with all add-ons disabled. If that solves the problem, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and possibly Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

The Skype extension for Firefox is known to cause phone numbers on web pages to disappear after loading initially - disable that extension.

Also, and although possibly not related to your problem, I have to remind you that the version of Firefox you are using at the moment has been discontinued and is no longer supported. On top of this, it has known unpatched bugs and security problems. I urge you to update to the latest version of Firefox, for maximum security, stability, performance and usability. You can get it for free, as always, at getfirefox.com.