JSON Bookmarks Viewer
View JSON formatted bookmarks in your browser. Just drag and drop the bookmarks file onto your browser window (javascript has to be enabled).
View demo.
Supported bookmarks format
Firefox
Open the bookmarks manager (Ctrl+Shift+B), select Import and Backup > Backup ... and save the JSON file to the location you selected.
Chromium
As far as I know, all Chromium based browsers store their JSON formatted bookmarks in a file called Bookmarks located in the user's profile directory (tested with the portable installations of Chrome, Opera and Vivaldi). I couldn't find any option to export the bookmarks from the browser, so you'll have to manually look for the file.