Using FF (43.0.4 on PC) Gmail contacts just wont open, it just hangs, contacts open just fine on Chrome & MS Edge.

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.

Interesting development. I've identified the popular ad-blocking add-on Ublock Origin as the trigger. With this disabled the issue goes away. Everything worked fine up to FF 43.0.4. There is no issue when using Chrome or MS Edge so it's looking more like a Mozilla issue than a Ublock Origin issue.

Thanks!