In general, mainstream plugins (like "Flash Player") required by
mainstream web applications (like Yahoo Messenger) are safe to
download, and will work well. You're not exactly treading new ground...

You won't be downloading the Windows versions - Firefox will pick an
implementation suitable for Linux / Firefox.

There is one "trap" to avoid. For some plugin functions, Firefox will
offer a choice of plugins. For your purposes, I'd recommend staying
with the most "mainstream" option you see. For Flash Player, that
will be the one from Adobe (I think; working from memory, here). (I
tried one of the others - Gnash, I think - it didn't work very well on
some of the web sites I needed.)

(Likewise, if you want to install a Java runtime environment, stick
with Sun Microsystems. I've tried - and abandoned - the "Iced Tea"
alternative.)

(Why are there several, you ask? Licensing. the Adobe / Sun licenses
are too restrictive for a GPL (General Public License) Linux
distribution, (but OK for individual end users). So the Linux
community is developing replacements under GPL. They are "works in
progress".)