The Open With submenu lets you temporarily override the default application for opening a file. For example, perhaps you normally open PDF files using Preview, but in a particular circumstance you want to use Adobe Reader instead. To do this, select the file and choose Open With ‣ Adobe Reader from the Record menu or the contextual menu.
The Always Open With submenu lets you change the default application for all files of a particular content type (UTI). For example, perhaps you want all your HTML files to open in BBEdit when double-clicked.
To change the default application just for a particular file, use the Reveal in Finder command, then choose Get Info from the Finder’s File menu and look in the Open with section of the info window.
EagleFiler will open Web archives in Safari even if you’ve chosen Firefox or Google Chrome as your default Web browser. This is because Web archives are a Safari-specific format. However, you can use the Open Source URL command to open a live version of the Web archive in your default Web browser.