JS_Add*Root
d js_addnamedstringroot) the address of the jsstring* variable to root opp jsobject ** (in js_addobjectroot and js_addnamedobjectroot) the address of the jsobject* variable to root rp void ** (in js_addgcthingroot and js_addnamedgcthingroot) the address of the jsstring* or jsobject* (not jsval) variable to root name const char * (in js_addnamedroot and js_addnamedrootrt) the name of the new root, or null.
... an entry for rp is added to the garbage collector's root set for the jsruntime associated with cx (or, in js_addnamedrootrt, the runtime rt).

JS_RemoveRootRT
this must have been passed to one of js_addroot, js_addnamedroot, or js_addnamedrootrt earlier.
...see also mxr id search for js_removeroot jsval_is_gcthing js_addroot js_addnamedroot js_addnamedrootrt js_dumpnamedroots, js_removeroot ...

JSAPI reference
since jsapi 31 js_addnamedgcthingroot obsolete since jsapi 31 js_remove*root obsolete since jsapi 31 js_removevalueroot obsolete since jsapi 31 js_removestringroot obsolete since jsapi 31 js_removeobjectroot obsolete since jsapi 31 js_removegcthingroot obsolete since jsapi 31 js_addroot obsolete since javascript 1.8.5 js_addnamedroot obsolete since javascript 1.8.5 js_addnamedrootrt obsolete since javascript 1.8.5 js_removeroot obsolete since javascript 1.8.5 js_removerootrt obsolete since javascript 1.8.5 js_mapgcroots obsolete since jsapi 19 jsgcmaprootfun obsolete since jsapi 19 js_dumpnamedroots obsolete since jsapi 19 local root scopes were another way of protecting objects from the garbage collector.

SpiderMonkey 1.8.5
jsautorequest jsautosuspendrequest jsautocheckrequest jsautoentercompartment js::anchor<> js::call obsolete apis js_clearnewbornroots js_enterlocalrootscope js_leavelocalrootscope js_leavelocalrootscopewithresult js_forgetlocalroot js_newgrowablestring deleted apis js_addnamedroot – use js_add*root js_addnamedrootrt – use js_add*root js_addroot – use js_add*root js_clearnewbornroots – no longer needed js_clearoperationcallback js_clearregexproots js_decompilescript js_destroyscript js_enterlocalrootscope js_executescriptpart js_forgetlocalroot js_getfunctionname js_getoperationlimit js_getscriptobject js_getstringbytes js_getstringchars js_isassigning js_leavelocalrootscope js_leaveloc...