Hi Nguyễn Trần Hải Đăng, the cookies that Google/YouTube sites use to recognize that you already signed in might be getting removed between sessions. Firefox's default setting is to allow sites to set persistent cookies and retain them indefinitely (until they expire). Many users customize their privacy settings not realizing the potential consequences, so I'll list out the usual things to check. '''''Why are my cookies gone?''''' '''(1) If you are visiting the site in a private window''' Cookies are not written to disk in private windows, so whether they are session cookies or have an expiration date after we're all gone, they will evaporate when the last private window is closed in your session. The two ways a site could open in a private window are: (a) Creating a private window in a regular session (for example, Command+Shift+p or right-click > Open Link in New Private Window) (b) Setting Firefox to use automatic private browsing on the Preferences page -- * Windows: "3-bar" menu button (or Tools menu) > Options * Mac: "3-bar" menu button (or Firefox menu) > Preferences * Linux: "3-bar" menu button (or Edit menu) > Preferences * Any system: type or paste '''about:preferences''' into the address bar and press Enter/Return to load it In the left column, click '''Privacy & Security''', then scroll down to the '''History''' section. Either of these will invoke automatic private browsing: * Firefox will: Never remember history * Firefox will: Use custom settings for history + "Always use private browsing mode" '''(2) If Firefox is set to store ONLY session cookies, and you have not made an exception for the site.''' You can check for this on the Preferences page, Privacy & Security panel, Cookies section. Make sure you do NOT have a checkmark for "Delete cookies and site data when Firefox is closed". If you do, you need to create "Allow" exceptions for sites where you want Firefox to accept persistent cookies. <img src="https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2019-05-01-08-10-39-73a6c7.png" width="500"> '''(3) If Firefox is set to Clear History When it Closes and that includes Cookies''' Most people do not have this setting, but you can double-check here: On the Preferences page, Privacy & Security panel, History section: make sure you do NOT have a checkmark for "Clear history when Firefox closes". If you want to use that feature with ''some'' types of data, use the Settings button to the right of it to confirm that Cookies are not being cleared. (Also, do not clear Site Preferences if you have made exceptions.) '''(4) If you use an add-on that modifies how cookies work''' There are extensions to manage cookie lifetimes and removal, including some that expire cookies for inactive tabs. There also are extensions that isolate pages in a container, which creates a separate cookie jar for the pages in that container which are invisible to pages outside that container. '''(5) If you logged out of the site''' The cookie identifies you to the site, but if your session ended, the site isn't going to start a new one automatically. So if you logged out on the site -- highly recommended for sites that have sensitive data or accounts you can't afford to have taken over -- then the site ''might'' pre-fill your user name on the login page, but you will need to sign in again. '''(6) If external utility or privacy software cleans browser cookies''' If you use CCleaner, Advanced SystemCare or other third party programs that touch browser data, set them not to touch Firefox data. '''(7) If your IP address is unstable''' Some sites link your identification to your IP address and require a new login if that changes. To minimize the potential for changes, you can check your Firefox connection settings on the Preferences page. In the search box at the top of the page, type ''proxy'' and Firefox should filter to the "Settings" button, which you can click. The default of "Use system proxy settings" piggybacks on your system settings (for example, Windows/IE "LAN" setting). "Auto-detect" can lead to a flaky connection. You may want to try "No proxy". Does any of that seem relevant to how your Firefox/system works?