abrowser + java
Obviously, I can't get abrowser to open any java applets.
I know that abrowser is essentially a rebranded Firefox. Why it's rebranded is beyond me. /It's like calling a pizza a pie./ I also think that the standard Java is not installed and is substituted with OpenJDK. Probably due to some "non-free" thing as usual.
I tried to psuedo follow the typical method of installing the java plugin for FF. But, sometimes you can't use a flathead drive to turn a phillips screw.
What version of Trisquel? If you are using 5.0 or 5.5, you do the standard "sudo apt-get install icedtea-plugin" which I believe uses OpenJDK 6 and installs it even if you have OpenJDK 7. Trisquel 4.0 and 4.5 only have OpenJDK 6. I wonder if it is a conflict between the JREs if both are installed. Hmm.
Trisquel's Web browser is rebranded because the code shipped by Mozilla is slightly modified (to remove any invitation to proprietary plugin) and Mozilla's trademark *forces* Trisquel to rebrand the result.
OpenJDK *is* the standard JDK. Oracle does not ship any other JDK working on GNU/Linux.
For your issue, and as t3g mentioned, it should only be a matter of installing the package named "icedtea-plugin", which as, as a dependence, the recommended version of the JRE, i.e., version 6.
*thumbs up* good answer.