Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

You might also try zooming out in case the zoom level is causing something to hide the scroll bar. You can zoom out using either:

• on the menu, the - button
• hold down the Ctrl key and tap the - key once or twice

Is this a commonly used a webmail site on which a volunteer might have an account?

You could also move your curser over where the scroll bar should be and it might appear. Its the new scroll bar. Which seems to be system wide. I had to install an addon in Thunderbird to get the old style scroll bar back. If you do a search in addons for the old scroll bar it might work in Firefox.