Firefox is usually known for its stability, compared to other web browsers. Sometimes when updating Firefox with plug-ins or extensions, it may happen that certain bugs happen when loading specific web pages.
When these types of errors occur, you would be prompted to close the Firefox.exe process.
Here is a method how to re-start Firefox:
To end the process, press CTRL + ALT + DELETE to get out of the Firefox application.
In the process tab right click on the Firefox .exe and end the process.
Open back Firefox and it will prompt you either, if you want to get back your previous session (open all the webpages you were on before the bug occurs) or if you want to open a new session.