Is there a firefox plugin which functions as an SVN client? I have checked on the official addon website. I really do not wish to go to the trouble of installing a standalone client, so is there currently a firefox plugin or some easy way I or someone else could make one? This would be very useful to me.
I found this firefox SVN extension, but it is basically just a shortcut to access TortoiseSVN.
By itself, Tortoise is integrated into the shell, and very easy to use.
For reading and downloading files from a repository, you just can use any web-browser and from Subversion 1.6 it even possible to see other revisions than HEAD. However you are not able to see log messages or changesets, also you are not able to checkout or commit files, however I doubt that a webclient is the right tool for it.
Addition: You can also use the webDAV functionality to write directly to files inside your repository (Autoversioning, see svnbook for details)
Also there are a lot of svn-webclients you can use within your browser Here are two projects: http://www.websvn.info/
Polarion WebClient for SVN supports also adding files and changing via download/upload cycle(do not know for websvn)http://community.polarion.com/index.php?page=overview&project=svnwebclient
From your comments it seems to me you're more interested in getting something that don't require an installation, and can be run from a flash drive.
One possibility for this is using the commandline client: http://www.open.collab.net/downloads/subversion/
It comes in a .exe installer, but you can just unpack it (I used 7-zip, not sure if that matters) and run it from any folder.
There's also several other clients listed here, maybe you can find one that suits your tastes: http://en.wikipedia.org/wiki/Comparison_of_Subversion_clients
Install the SVN plugin in IE. Then Install the IE Tab plugin.
In Tools, IE Tab Options, select "always use current engine to open a new URL". Browse to any page, right click and select "open in IE Tab". Now enter the svn:\ url and display the page. Bookmark the page.
To open the svn page again, right click on the bookmark and select "open in IE Tab".
If you always use bookmarks, You may de-select "always use current engine to open a new URL"