Just hit your F7 key while in Firefox. It’s that simple! What it does is toggles caret browsing mode. When that mode is turned on, using the arrow keys and the pageup/pagedown keys will move the little caret/cursor around the text in the browser window instead of scrolling the page like you probably want.

Yeah, so somehow this mode got toggled for you. In Ubuntu it may be the default, at least for me.

Here’s where I found this answer.

Note: In my case using Ubuntu Dapper I located these “locked” files in this directory:
/home/chiefinnovator/.mozilla/firefox/q5ocj63r.default
More generally that pattern is:
~/.mozilla/firefox/[some random looking letters.default]

And the quickest way to delete this files is to open up a terminal and run these two commands (altered for your respective locations):
$ sudo rm /home/chiefinnovator/.mozilla/firefox/q5ocj63r.default/lock
$ sudo rm /home/chiefinnovator/.mozilla/firefox/q5ocj63r.default/.parentlock

So you’re trying to connect to your Router’s web-based administration console and you get this strange error:

You have received an invalid certificate. Please contact the server administrator or email
correspondent and give them the following information:

You certificate contains the same serial number as another certificate issued by the certificate
authority. Please get a new certificate containing a unique serial number.

Here’s a screen shot of this error:

To Fix This:
What you need to do is clear out the certificate for the router in your browser. Here is how to do it in Firefox, and some general certificate info here. (If you’re using I.E. you probably have bigger problems ) If you have a different browser than Firefox, best to just do a search for deleting certificates in your browser.

After doing that. Connect to your router and be sure to turn off whatever option you checked to require https. Or just follow this process everytime.

This advice may also apply for any other time you get this error. Though I don’t know much about certificates so you may want to look into the security implications of this error as well.

My search queries:

1. “You have recieved an invalid certificate” + linksys
2. “You have received an invalid certificate” + linksys
3. Firefox clear certificates
4. Firefox delete certificate

To change this for versions of Firefox <1.5, this Mozilla Knowledge base article tells us to:

Type about:config into the Location bar and hit Enter. Find the entry for security.checkloaduri and change its value to false by double-clicking it. Then, restart your browser.

For newer versions of Firefox just follow the complex sounding instructions from that article. Alternatively this Firefox extension: Policy Manager may give you an easier go at it.

Here are a few other helpful websites for this issue:
Linking to local file under Firefox
file Protocol

Unfortunately I still have not figured out how to have a user be able to click on a link to a local resource in Firefox and have Windows Explorer open up that folder (i.e. the way it happens in Internet Explorer). I’ll keep looking.

Here is my massive list of search terms:

1. file:/// protocol firefox
2. file:/// protocol
3. hyperlink file protocol
4. hyperlink to path
5. Firefox does not follow links from http resources to file resources
6. firefox hyperlink file protocol
7. firefox launch OR open windows explorer directories OR folders
8. firefox browse windows directories OR folders
9. clicking on link opens windows explorer
10. hyperlink explorer windows file
11. hyperlink explorer windows
12. hyper link explorer windows
13. windows file link

Illegal Operation in Plug-in
————————-
The Plug-in performed an illegal operation. You are strongly advised to restart Firefox.
[ ] Don’t show this message again during this session.
—-
OK

Here’s a picture.

This forum post sounds like the answer so go knock yourself out. (It sounded like too much work for me this morning so I’m going to just keep clicking the OK button )

Update:
I tryed the instructions from the link above and didn’t have any luck, but here are a few other advice pages to try:
(translated from French!)
Problem with Plugin “Shockwave Flash”
The plugin carried out an illegal operation

Update Update:
Give Hugo’s advice in the comment below a try. It certainly sounds promising.

Update Update Update:
Lukpuk comments below that it may be a virus. He reccomends to run msinfo32.exe and find something weird in loaded modules or autorun. Then try to delete a virus in windows/system32/ (He used Hijack this) during the boot.