Go thru your add-ons list. Was this done using an add-on?

I use; https://addons.mozilla.org/en-US/firefox/addon/thumbnail-zoom-plus/ Thumbnail Zoom Plus Shows full image when you hover over a thumbnail. Works with Amazon, Baidu Images, Bing Images, Facebook, Flickr, Google+, Google Images, IMDb, LinkedIn, Netflix, Pinterest, Reddit, Twitter, Yandex, YouTube, Wikipedia, Yahoo Images, & many more.

That add-on just initiated another picture the same seize. No magnification of any kind. The initiated new picture was on the same screen, just next to the original.

Please note: The problem is not solved. I incorrectly clicked the "solved" button

Is this magnifier part of the website design or is this a feature that is added by an extension that may not work properly anymore?

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Check this web page: http://www.imagebam.com/image/94fffc477981583

When you move the cursor on the image a + sign appears. When the cursor is placed on the image and clicked, the image is magnified. That now does not happen with Firefox. It used to on older version and still does on Microsoft Edge.

In that case it is done via JavaScript :

function() {
  $(img).toggleClass("zoomed");
}

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block (wanted) content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then always make sure such extensions do not block (wanted) content.

• make sure that your extensions and filters are updated to the latest version

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems