That looks like a symbol used by the extension Auto Tab Discard. You can choose in the extension settings NOT to use it. I find that it occasionally uses it even if the symbol is not enabled.

If you don't have that extension installed, it may be another extension or your theme.

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems (also deals with hardware acceleration) https://support.mozilla.org/en-US/kb/troubleshoot-issues-related-to-add-ons

That looks like a symbol used by the extension Auto Tab Discard. You can choose in the extension settings NOT to use it. I find that it occasionally uses it even if the symbol is not enabled. If you don't have that extension installed, it may be another extension or your theme.

Thank you so much. I found that I already installed that extension -I don't remember why I install it frankly-, I disable it and every thing back to normal.