Does Firefox show the Open/Save/Cancel dialog, or try to use some other plugin to handle the file?

Sometimes this problem is caused by the web server indicating to Firefox that it is a content type for which the plugin is not registered. However, checking that is a little tricky... If you open the web console (Ctrl+Shift+k) in the lower part of the tab and then click the link to open the TIFF file, usually a line will appear in the console. When you click it, you can view the headers exchanged by Firefox and the server, and the Response Headers should containing a Content-type. If the TIFF opens in a new tab, this trick doesn't work.

To see what content types AlternaTIFF is registered for, you can type or paste about:plugins in the address bar and press Enter. You can use find in the page (Ctrl+f) to locate AlternaTIFF. If the site is sending a content type that isn't on the list, there's your trouble. And we'd need to think about a possible workaround.

The second website seems to be able to use other ways like a "Quick Document Viewer" (PNG) and "PDF Viewer" to show the document. You can also download the document as a TIFF file (Content-Disposition: attachment;filename="xxxxx.tif" ). There is some script running that checks the navigator.plugins array for the AlternaTiff plugin (pluginFound = detectPlugin('AlternaTIFF');).

In response to both answers - When I open the box under Action it offers 1 application and then others. Choosing others makes several other applications available, but no plug-ins, so I couldn't choose what I wanted. Yes, the 2nd site does let me use other apps, and somehow earlier today I managed to open the tif files in windows photo viewer and see what I wanted, so I've got a good workaround. Thank you both for your prompt response.