Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. The Firefox cache temporarily stores images, scripts, and other parts of websites while you are browsing.
Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies, do the following:

1. Tap the menu icon located at the top right corner. This is the icon with 3 bars. On older Android devices you'll have to press the hardware menu key and then tap More.
2. Tap Settings.
3. After that, you will be taken to the settings screen. In the settings screen, look under the section Privacy & Security and select Clear private data.
4. You will then be taken to a list of what can be cleared. Select the following 2 for deletion:
   • Cookies & active logins
   • Cache
5. After those have been selected, tap the Clear data button to actually clear the cache and cookies.

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In Firefox for desktop systems, there are cookie viewing dialogs where you can remove individual cookies, or remove all of one website's cookies.

In Firefox for Android, I haven't seen a dialog offering that level of control. You can clear all the cookies using the steps in this article: Clear your browsing history and other personal data on Firefox for Android. However, to target individual cookies you probably would need an add-on (or deeper knowledge than I can bring to bear).