Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Is this a Flash-based designer? Usually if you right-click content in a Flash control, there is a distinctive context menu different than the one you get on regular web pages.

(I think on Mac, if you can't right-click, Ctrl+click gives you the same result.)

If a layout gets out of whack within Flash, it may help to switch between full screen and regular views to have Flash repaint the layout. To switch Flash, check for that option in a bar at the top or bottom of the designer, or on the right-click context menu. To switch Firefox, use the View menu > Full Screen. To exit Firefox full screen, slide the mouse pointer to the very top of the screen and right-click on a blank area of the tab bar or on the new tab ("+") button. If Flash is full screen, you probably need to exit that first (try pressing the Esc key).

On MacOS X 10.9 (Mavericks) Firefox relies on the system setting in the OS for the scroll bar. Please see this thread for how to modify the system setting: why do the scroll bars quickly dissapear in firefox on OSX 10.9?