Gentoo let the user installing a few Firefox and Thunderbird plugins via the package manager and people often consider it is useless. Actually, there are at least two reasons why it can be useful.
The first one is obvious : when installing a plugin with the package manager, you are sure it will be available for every users (except if it is manually disabled by the user). For example, you can install noscript for security reasons directly by enabling restrict-javascript USE flag.
The other reason is the compilation. Indeed, most plugins are platform-independent but some of them are not and those ones are not guarantee to work on your computer. For example, you can install Firefox on a lot of different platforms. If you stay with GNU/Linux, you can install Firefox 3.5 on alpha, amd64 (x86-64), arm, hppa, ia64, ppc, ppc64, x86[1] . This is true at least with Gentoo. It goes without saying if a plugin has a platform-dependent part, it will not provide a binary for all these platforms.

Concretely, I tried to install Weave 1.0 beta 1 and 2 from addons.mozilla.org. When launching Firefox, I got this output in the Weave Activity Log view:

2009-11-25 12:34:02	Service.Main         INFO	Loading Weave 1.0b2 in 5 sec.
2009-11-25 12:34:08	Engine.Bookmarks     DEBUG	Engine initialized
2009-11-25 12:34:09	Engine.Forms         DEBUG	Engine initialized
2009-11-25 12:34:10	Engine.History       DEBUG	Engine initialized
2009-11-25 12:34:10	Engine.Passwords     DEBUG	Engine initialized
2009-11-25 12:34:10	Engine.Prefs         DEBUG	Engine initialized
2009-11-25 12:34:10	TabTracker           DEBUG	Failed to load json: this.changedIDs is undefined JS Stack trace: ([object Object])@trackers.js:125 < Utils_jsonLoad("weave/changes/tab_tracker.json",[object Object],(function (json) {for (let id in json) {this.changedIDs[id] = 1;}}))@util.js:559 < T_loadChangedIDs()@trackers.js:123 < T__init()@trackers.js:76 < TabTracker__init()@tabs.js:356 < TabTracker()@tabs.js:345 < ()@engines.js:161 < Engine__init()@engines.js:179 < _init()@engines.js:303 < TabEngine()@tabs.js:56 < EngMgr_register(TabEngine,5,[object Array])@engines.js:113 < EngMgr_register([object Array])@engines.js:106 < WeaveSvc__registerEngines()@service.js:379 < _onStartup([object Object])@service.js:278 < notify([object XPCWrappedNative_NoHelper])@util.js:624
2009-11-25 12:34:11	Engine.Tabs          DEBUG	Engine initialized
2009-11-25 12:34:11	Service.Main         INFO	Resetting client syncID from _onStartup.
2009-11-25 12:34:11	Service.Main         INFO	Mozilla/5.0 (X11; U; Linux ppc; en-US; rv:1.9.1.4) Gecko/20091109 Gentoo Firefox/3.5.4
2009-11-25 12:34:11	Service.Util         WARN	Component @labs.mozilla.com/Weave/Crypto;1 requested, but doesn't exist on this platform.
2009-11-25 12:34:11	Service.Main         DEBUG	Crypto check failed: TypeError: Svc.Crypto is null
2009-11-25 12:34:11	Service.Main         ERROR	Could not load the Weave crypto component. Disabling Weave, since it will not work correctly.
2009-11-25 12:34:11	Service.Main         INFO	Weave Sync disabled

If you unzip the xpi file, you will found a platform/ directory which is containing platform-dependent files. If we stay with GNU/Linux, we have Linux/, Linux_x86_64-gcc3/ and Linux_x86-gcc3/ directories. If we launch 'file platform/Linux/components/WeaveCrypto.so', we got:

platform/Linux/components/WeaveCrypto.so: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked, not stripped

So, x86, x86-64 and arm are supported, not surprising. But my old Powerbook G4 (PowerPC) is not. Actually, it would be if I was using MacOS X instead of GNU/Linux.

I don't know if addons.mozilla.org should specify the supported platforms for platform-dependent plugins because arm, x86 and x86-64 are covering most of the target systems. Actually, as far as I know PowerPC is fully supported only by Gentoo and Debian. Anyway, if you are using an unconventional platform, you may want to compile your own weave plugin and probably a few others.

Luckily, weave has just been updated to 1.0b2 in the Gentoo tree :)

Notes

[1] mips is working with Firefox 2.0.0.19 and sparc with Firefox 3.0.11