Either Firefox is crashing every time you close it (seems unlikely) or your setting is being overridden. The most common way it gets overridden is if external software creates or edits an optional settings file named user.js. You can use the steps in this article to check for that: [[How to fix preferences that won't save]]. By default, Windows will hide the .js extension, which makes it difficult to work with .js files (for example, to rename them) accurately. You can change this Windows setting using the steps in this article: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions Was that it?