Details like websites remembering you (log you in automatically) are stored in a cookie.

• Create a cookie 'allow' exception to keep such cookies, especially in case of secure websites and when cookies expire when Firefox is closed.
• Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

Make sure that you do not run Firefox in permanent Private Browsing mode (Never remember history).

• https://support.mozilla.org/kb/Private+Browsing

Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Already tried clearing cache and creating a cookie allow exception, as well as clearing the problem website's cookies. I never use Private Browsing because I've not found the purpose of it yet. (I know what it does, just not sure of the advantages of using it.)

The problem was in the cookies files in the Firefox profile. I followed your instructions for and I'm no longer being logged off of YouTube. Thank you so much. This is the sort of thing I would have never figured out on my own. I'll rate your comment as Helpful, but I'm not sure if that will close this thread or not and, if I may, I'm having two other problems with Firefox:

1) Buffering. Even short YouTube videos (as well as videos on other sites) often play for just a moment and then stop to buffer for awhile... play again for a brief moment, then buffer again... repeatedly. With Safari, I can click and play without any issues and rarely any buffering at all.

2) Using the Back Button or swiping my Magic Mouse to go to the previous screen. It rarely allows me to do this and I have to re-enter the site completely.

Any suggestions here?

I should clarify that I have the problem with the Back Button (and swiping Back with the Magic Mouse) only when I'm watching videos. There are times when swiping or clicking on the Back Button does absolutely nothing. Other times, all it does it cause the screen to flash and take me back to the beginning of the video. THEN, when I click/swipe again, it goes back to the previous page.

Again, this isn't an issue with Safari. But, I want to get these bugs ironed out because I'd like to use Firefox as my default browser.

Anti-Virus, Disk Cleaners, and other such programs may delete cookies, history, and other not wanted things. Open their options and disable ANYTHING having to do with the browsers.

Firefox also has this option.

Thank you, FredMcD. I'm not running any anti-virus, disk cleaners or other such programs. (Should I? I haven't seen any need, but I'm not an expert.)

All of my Firefox preferences and options have been checked, too. It ended up being corrupt cookie files in the Firefox profile. I deleted them and it fixed that problem.

I spoke too soon. The problem with YouTube is back. I clicked another comment to rate it and the same problems happened.

Forget it. It's too frustrating. This shouldn't have to be this difficult. I switched to Mac because of these kinds of problems with Microsoft. Safari works just fine so I'll stick with that.

I don't have a lot of programs, extensions, etc., that I've downloaded that could be causing these problems. I have Adblock Plus, WOT and Ghostery. They cause no problems with Safari. I've had issues with Firefox for two years. Every time I build up the patience to work with it to try to fix the problem again, it never gets fixed. I understand that it's difficult to diagnose someone's computer problems and that people are only trying to help and are doing their best. Volunteering their time, even. For that, I'm grateful. But, I'm also frustrated. I've even completely reinstalled Firefox three times over the last couple of years. I end up having to reset everything, redo my bookmarks, etc., It's just not worth it anymore. I'm deleting Firefox for the last time.

This is just my feedback for Firefox, for whatever they think it's worth.

JackParker said

Thank you, FredMcD. I'm not running any anti-virus, disk cleaners or other such programs. (Should I? I haven't seen any need, but I'm not an expert.)

Yes, Please. Many files are created by running programs. And sometimes a file ends up forgotten, taking up disk space.

And Anti-Virus can stop a bad program and web intruding signals, that do show up when someone tries to attack you system. Have you been watching the news of late?

JackParker said

I spoke too soon. The problem with YouTube is back. I clicked another comment to rate it and the same problems happened.

Please try this; You may have a corrupt cookies.sqlite file.

Type about:support in the address bar and press enter.

Under the page logo on the left side you will see Application Basics. Under this find Profile Folder. To it’s right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the file. Then rename or delete it. Restart Firefox.