Is the circle with a slash through it an icon in the address bar? (Firefox may shown the Tracking Protection shield with a slash through it if the feature is turned off for the current site, or it may show the lock icon with a slash through it for an insecure connection, etc.)

Or is it part of an error page? If it's hard of a page, is there any other information about what is going wrong?

Or it is a symbol next to the mouse pointer indicating you cannot click or drag/drop or do some other mouse action?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot