iMac often changes Preview for jpg files to Firefox or other

Select a .jpg or .pdf file. Press Command-I to open the Get Info window and go to the Open With panel. From the Default App drop-down menu locate and select Preview then click on the Change All button. Repeat for each extension you prefer Preview to open. If Preview isn't found on the drop-down menu then at the bottom of the list select Other. Locate Preview in the File viewer, select it and click on Open button.

I suggest you also rebuild the Launch Services database.

Rebuild LaunchServices Database

Open the Terminal application in your Utilities folder. At the prompt paste in the following command in its entirety:

find /System/Library/Frameworks -type f -name "lsregister" -exec {} -kill -seed -r \;

Press RETURN. Wait for the Terminal prompt to return after which you can quit the Terminal.

I've noticed something strange. Maybe this will help finding the cause of the problem.

When I drag a JPG from Firefox to the desktop the default application for viewing/editing JPG files changes to Preview. I tried Chrome and Safari too and then the default app for JPG files stays the same (Photoshop in my case). It is easy to reproduce. Change default app for JPG to Photoshop (⌘+i and Change All...). Drag an image form FF to desktop and t reverts back to Preview. So my guess this is really a Firefox bug.

Before Firefox drag to desktop:

After Firefox drag to desktop:

Thanks for posting. It is a known bug in the macOS High Sierra.

However what is interesting is your macOS is El Capitan. This would be worth reporting.

Apple Feedback http://www.apple.com/feedback/

With your same Apple ID you can sign up for a free Developers Account and start a conversation with Apple engineers

Bug Reporter https://bugreport.apple.com/

Yours is one of many variations on the theme. Dragging pics from other browsers is known to change the selected default opening as well, many post here in the ASC along these line—in High Sierra, I do not remember this being an issue in El captain before.

I also have the problem with jpegs opening with Firefox instead of Preview. Pdf files would open in iBooks. Additionally, clicking on links in retailers' emails opened Firefox. I have noticed when this is occurring, if I go to apple>system preferences>general, the default web browser window is blank, with no choices available with the up-and-down arrows. My temporary fix is to re-start the computer. After re-start, the default web browser window shows Safari, and the types of files mentioned open in preview. Email links open in Safari again. Everything acts normally for awhile, but the problem comes back. This has been going on for a couple of months now.

bjprc, I am wondering if you have tried the fix to "rebuild launchServices database" suggested by Kappy? If so, could you please post here with your results? I am considering trying that step too, but like you, I am always scared to try anything that could have unintended consequences. I am on El Capitan 10.11.6, on a mid-2014 21.5" iMac.

I'm not sure it will help, but try running this program in your normal user account, then copy and paste the output in a reply. The program was created by etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. On the screen with Options, please open Options and check the bottom 2 boxes before running. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown. You can run the report for free at least once, but if you run it several times, at some point it will ask you to pay a license fee.

Etrecheck – System Information

Re: Mac's Grab keeps opening my screenshots & dragged desktop images in Grab instead of Preview. PLEASE HELP

In this case, just copy and paste the red line. Not much can go wrong.

I would try Kappy's suggestion. As posted, nothing much can go wrong, and Kappy has a very good working knowledge of how to fix problems.