As you're already surmising in your question, BibTeX's structure and design isn't great for citing items that aren't some form of literary publication (such as books, articles in journals, working papers, etc). BibTeX does have a catch-all entry type called @misc, but you'll find yourself dealing with a lot of compromises if you take this approach; in cases such as the one you're describing, one size (or bibliographic entry type) does not fit all.
All is not lost, however: You could use the much newer biblatex package, which styles itself as "a complete reimplementation of the bibliographic facilities provided by LaTeX". As such, it recognizes many more entry types (in addition to supporting all entry types recognized by BibTeX). Some of these entries that may be of relevance to you: online (any online resource), patent, artwork, audio, image, jurisdiction (court decisions), movie, performance, review, and video, to name just a few.
All this extra functionality comes at a price (of course!): because biblatex --especially when run with the dedicated background program called biber -- is far more powerful and flexible than the LaTeX/BibTeX combo that's been around for more than 20 years, the learning curve is also steeper.
You can set up Jabref to automatically import a reference from Firefox into the current database, but it's somewhat arcane. Here is my solution under Linux:
1) Select Options -> Preferences -> Advanced -- and check "Listen for remote operation ..." I don't think it matters which port.
2) Create a small bash script (text file) named "jabref-import" that looks like this:
#!/usr/bin/env bash
java -jar ~/local/jabref/JabRef-2.8.1.jar -i --importToOpen "$*"
Replace "~/local/jabref/JabRef-2.8.1.jar" with the path to your Jabref .jar file on your machine. Or if you have a working executable called "jabref", you can replace everything before the "-i" with "jabref". Just make sure your executable accepts command-line options (mine didn't).
In Ubuntu 13.04, the following variant of the script works:
#!/usr/bin/env bash
jabref -i --importToOpen "$*"
3) Make the file executable:
chmod ugo+x jabref-import
4) Make sure Jabref is already open. Go to Firefox, download a citation file. It could be a .bib or .ris or .ref or whatever. Select the "Open with..." option in the dialog, and select the jabref-import executable that you just made. The import dialog should pop up in Jabref with your citation.
Best Answer
You can use arbitrary keywords that are not yet defined. They will be ignored by
bibtex.In JabRef, you can add those fields in
Options/Set up general fields.