In some instances, Firefox may not remember changes you've made to your toolbar settings or certain other configuration changes, like window size or position.
This article explains how to fix the underlying problem that may result in one or more of the following symptoms:
Here are two ways that you can solve the problem:
You may have a corrupt xulstore.json file in your Firefox profile. You can delete this file so that Firefox can create a new one.
Click the menu button , click help and select Troubleshooting Information. The Troubleshooting Information tab will open.
If you don't want to try the first solution or if it didn't work, the Firefox refresh feature will automatically restore default toolbar settings and browser window size or position, and will also remove add-ons (extensions and themes) that could be interfering.