Ubuntu :: System Text Replaced With Squares

Yesterday, while surfing a web forum with ~6 tabs open and music playing, I noticed that the top of my Firefox window, next to the minimize/restore/close buttons, was displaying the wrong name. It was displaying my second tab while I was on my fourth, for example, and this error continued even as I switched tabs, picking a tab for the name at seemingly random. The menu bar next to the "click here to hide all windows and show the desktop" button continued to show the right name, however. Within a minute or so, the browser started to hang, my computer started to slow, and the top of the browser became entirely obscured. I was eventually forced to power down the computer by use of the power button, as I was unable to access the shut-down menu.Upon booting up again, I noticed that the computer took longer to get to log-in. Once I did log in, I noticed that the text at the top of the window now consisted entirely of squares, with about as many squares as there would be proper characters normally. Furthermore, this had spread to other aspects of my system. I have all current updates.

Examples (attached pictures should illustrate):

- When in the File Manager, there are tabs that show your movement through the system; for example, if I start by opening Home, then Pictures, it'll show my Home folder and Pictures there. The currently-selected tab has the erroneous display of squares, but the non-selected ones do not.
- The shutdown menu has squares where the names of the individual options should be and at the top window, but the descriptive text below continues to function normally.
- The update manager has squares at the top line of the "loading" prompt, the top line next to the update manager icon (just above "The package information was last updated X hours ago"), and the name of each individual update consists of squares, in addition to the text just below "changes for the versions: 2.2... etc" text but above the changelog.
- The "Users Settings" options menu has squares at various areas, such as the name of the user, account type, and password. (The area with the name and colon, such as "Password:" is correct, but the area after the colon is all squares.)
- Black box system notifications - for example, the name of the wireless to which I have just connected - have the first line made into squares.
- Authentication prompts - for the package manager or update manager, for example - have the first line as squares, but the second works normally.
- The package manager seems completely fine, other than the aspects of it previously mentioned that extend to other applications (the name area at the top of the window, the authentication prompts).

(Looking back, I actually meant "rectangles" when I said "squares." Oops; apologies for any confusion caused.)It seems obvious that SOMETHING has become corrupted, but I'm not educated enough on the subject to identify and solve the problem, or to know much of the standard procedure for this sort of situation.