what is the proper program used to open files like add-ons etc?

I think I messed something up in Firefox.I was in the process of backing up some files like bookmarks.I had done the following-clicked on Firefox,troubleshooting and show folder.I found and back up bookmarks but had trouble when it came to trying to do places etc. and now some of the files are showing Acrobat to open them. What is the default for these files and how do I switch back? Thanks

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