Default apps keep changing suddenly

I have a MacBook Pro running El Capitan 10.11.3. For the past couple of months, since Firefox decided to update itself to something brand new, every time I open a jpg or a pdf, it opens in something other than Preview which is and has always been my default application for these. I've set my default in Firefox preferences to use Preview and it doesn't change but I still get something other than preview.

They open in Firefox. They open in Adobe Digital Editions. They open in iBooks since last week. And today, suddenly jpgs open in Color Sync Utility. But they don't open in Preview - which is what I have always used since Macs had Preview.

Text Edit rtf docs sometimes randomly open in Open Office and then when I try to open something I made in Open Office, I either get a 'can't open it because that application doesn't exist' OR a 'do you really want to open Open Office' because apparently, I've NEVER opened it before.

When I simply go to Open With, the default is something other than Preview and I have no idea how to change that. Besides, the default application changes constantly and randomly.

Does anyone know why this is happening? It only started after Firefox updated itself about 2 months ago or maybe a little more. I'm not going to use Chrome and I'm thinking of going back to Safari.

But I'd like to know of anyone else is having these poltergeist issues and if you've found a cause and/or a fix. Is this related to Firefox? Has it messed up my system? Or is it a system issue that only coincidentally happened when Firefox changed up?

Thanks for any feedback and please don't suggest something in technogeek. I need English. Thank you.