A .png file is usually a static image. In this case, it's an icon. What you want to find in the Page Info Media panel is something like the attached (downsized for posting). The video you are playing most likely loaded near the end of the sequence, so check toward the bottom of the list.

Edited to add comments from two other posts:

Summary: To Save a Video Using the Page Info dialog

Call that up using either:

• right-click a blank area of the page and choose View Page Info
• (menu bar) Tools menu > Page Info
• click the padlock or "i" icon to the left of the site address, then the ">" icon, then More Information

At the top of the Page Info dialog, click Media.

Scroll the media list to the first item of type Video, select that, then click Save As.

Note: It can be hard to spot. Try sorting Videos to the top of the Type column by clicking the Type column heading twice (first click should sort A-Z, second click should reverse the sorting Z-A).

If Firefox doesn't recognize that there is a video in the page, it could be in an embedded document instead of directly in the page. (For example, how YouTube videos are embedded on other websites.)