What does the blue triangle mean?

That seems to be an icon related specifically to whatever app is opened so as mention is very hard to know what it means without any context, or details including what App this is coming from.

If you can share what app this is happening in, we may be abet determine what the icon means in the context of said app.

Your screenshot is heavily cropped - removing all context. Without more information as to the App being used - and when and the circumstances the icon appeared - it is very difficult to speculate or provide a definitive answer.

If you can provide more information - and ideally a larger screenshot (if necessary using markup tools to obscure any sensitive or personal information) - contributors here may be better able to offer more insight.

If this appears inside the Firefox app on its ... menu it usually related to something in that menu. If you tap on the ... menu it should show the same or similar icon next to the item that needs your attention.

Sometimes related to the account being used for Firefox data sync.