there is a yellow triangle in the upper left corner

A gray padlock with a yellow or orange triangle on it usually indicates that one or more images in an HTTPS page were retrieved using an HTTP address: in other words, insecure display content in a secure page.

This article has more information on the mixed content icons, and what is blocked and what is not: Mixed content blocking in Firefox.

Are the triangles on pages that have a high security concern, for example, where you are logging in, entering confidential data, or reading sensitive information?

If so, that would be unusual and cause for further investigation.

If not, for example, if it's a news article, image search, or general email, then I wouldn't be too concerned about the triangle.

By the way, I'm assuming this problem was in Firefox for Windows. Your post ended up under "Focus by Firefox" which I think is an add-on for the Safari browser on Mac iOS devices. A moderator can move your post if needed.