I would like to copy multiple cells from Excel to Firefox, and I am wondering if this is possible?
For example, a website contains fields such as: Address, City, Postal Code, etc. below one another. I would like to enter that information in excel, and select/copy/paste all rows at once to Firefox instead of doing one at a time.
It seems like Firefox needs to detect that a "tab" entry is needed after each row. Are there any add-ons or scripts to achieve this?
There are a multitude of form fillers that do this. Trying to do it with Excel is reinventing the wheel with tools that aren't designed for this purpose.
Some of the form fillers are browser add-ons and some are stand-alone programs. Firefox, itself, will do this to a limited extent if the feature is turned on. Almost any form filler will handle a single entry box on a page, or several if the boxes contain an identifying label that can be recognized programmatically. The limitation is that the simple form fillers usually can't handle multiple, unidentified boxes, and some forms purposely suppress auto-fill on some fields.
The more robust form fillers allow you to also build a database of common entries. When you need to manually enter one of those you select it from the list.
Since the capabilities of the offerings vary and I don't know your exact needs, it's difficult to recommend a specific one. However, a Google search, or an add-ons search, for "form fillers" will give you endless options, with many that are free. From your comment, it looks like you found that the Autofill Forms Firefox add-on does what you were looking for.