Firefox 43 introduced a security feature that prevents unsigned third-party add-ons like Webvisum from working. Follow these steps to get it to work with Firefox 43:
1. Open Firefox 43 and in the address bar type the below line as seenn with no spaces:
2. You will get a Firefox warning saying, "This may void your warranty." Click on the button that says, "I'll be careful, I promise."
3. Tab once to the list of 700 or so items and press the letter X. You are looking for the below item:
4. When this item is highlighted, press the context key or right click and choose toggel or press T (same thing.)
5. The highlighted value should change to false as in:
6. Then press control + W to close the window.
when you run Firefox again Webvisum should work.