I have already written a few posts on PDF and one more post on this tag. We do anything on a monitor using three types of interfaces; (1) Desktop (2) Online (3) Browser based. My previous posts were on Desktop and online interfaces for PDF files and this post is for browser-based interface or something similar.
Working on PDF files or any activity related to PDF files we use mostly Desktop as a medium but browsers also cam enable us to do many of tasks using browser interface.

So this post is on Firefox add-ons for PDF files, using these add-ons you can download, convert, print, edit and view PDF files.
PDF Download
You can us this add-on to download PDF files. You can view PDF files directly in your FF browser.
Web to PDF
This add-on allow you to convert a web page in to a PDF document within your browser.
PDFescape
PDFescape allows you to open and edit PDF files and forms online.
Print PDF
Windows and Linux users can Print a HTML page to PDF. This works only with Firefox 3.
Aero PDF
Using this add-on you can search PDF files with one click within your browser.
EBook PDF Search Engine and Viewer Toolbar
It is a PDF ebook searched and viewer toolbar.
Google Doc Viewer
If you have installed this add-on, it will open any DOC, DOCX, PDF, PPT and TIFF using Google Docs.
Only PDF
This add-on searches PDF files fast using Google search engine.
Loop to PDF
You can convert and combine files to PDF.
PDFit
You can convert a page to an image of a PDF file.