It copies the files to the phone's sdcard Download folder. So /sdcard/download/

However the files on Firefox for Android are not compatible with Firefox desktop. What are you trying to do?

I want to have archive of my mobile favorites locally. I found profile saved in /sdcard/mozilla_profile/ , not in download. Below is content of mozilla_profile folder. Where can I find bookmarks?

2014-06-28 14:13 <DIR> adblockplus 2014-06-27 23:51 46,178 addons.json 2014-06-27 23:51 87,716 blocklist.xml 2014-06-27 23:50 3,850,240 browser.db 2014-06-27 23:51 32,768 browser.db-shm 2014-06-27 23:50 4,148,872 browser.db-wal 2014-06-28 14:14 <DIR> Cache 2014-06-27 23:50 360,448 cert9.db 2014-06-27 23:51 83 cert_override.txt 2014-06-27 23:50 214 compatibility.ini 2014-06-27 23:51 229,376 content-prefs.sqlite 2014-06-27 23:51 1,277,952 cookies.sqlite 2014-06-27 23:51 32,768 cookies.sqlite-shm 2014-06-27 23:51 1,377,296 cookies.sqlite-wal 2014-06-28 14:14 <DIR> crashes 2014-06-27 23:50 163,840 downloads.sqlite 2014-06-28 14:14 <DIR> extensions 2014-06-27 23:51 47,978 extensions.json 2014-06-27 23:50 196,608 formhistory.sqlite 2014-06-27 23:51 303,104 health.db 2014-06-27 23:51 25,136 health.db-journal 2014-06-27 23:51 523 history.xml 2014-06-27 23:50 294,912 key4.db 2014-06-28 14:21 0 list.txt 2014-06-27 23:50 169 localstore.rdf 2014-06-27 23:50 0 lock 2014-06-28 14:14 <DIR> minidumps 2014-06-27 23:51 884,736 netpredictions.sqlite 2014-06-28 14:14 <DIR> OfflineCache 2014-06-27 23:50 65,536 permissions.sqlite 2014-06-27 23:50 891 pkcs11.txt 2014-06-27 23:51 433 pluginreg.dat 2014-06-27 23:50 8,138 prefs.js 2014-06-27 23:51 3,190 profile_info_cache.json 2014-06-27 23:51 578 recommended-addons.json 2014-06-28 14:14 <DIR> safebrowsing 2014-06-28 14:14 <DIR> saved-telemetry-pings 2014-06-27 23:51 247 search-metadata.json 2014-06-27 23:51 35,427 search.json 2014-06-28 14:14 <DIR> searchplugins 2014-06-27 23:51 7,594 sessionstore.js 2014-06-27 23:50 327,680 signons.sqlite 2014-06-27 23:51 2 snippets.json 2014-06-28 14:14 <DIR> startupCache 2014-06-28 14:14 <DIR> storage 2014-06-27 23:51 53,248 tabs.db 2014-06-27 23:50 32,768 tabs.db-shm 2014-06-27 23:50 436,752 tabs.db-wal 2014-06-27 23:50 27 times.json 2014-06-27 23:51 154 urlclassifierkey3.txt 2014-06-27 23:50 7,602,176 webappsstore.sqlite 2014-06-27 23:51 32,768 webappsstore.sqlite-shm 2014-06-27 23:51 1,311,712 webappsstore.sqlite-wal 2014-06-27 23:51 1 _CACHE_CLEAN_

They are stored in bowser.db. Unless you have root on the device there is no way to make use of the backup.

You should consider Firefox Sync. https://support.mozilla.org/en-US/kb/how-do-i-set-up-firefox-sync

"Firefox Sync" - that is exact reason, why I want capability to archive mobile bookmarks. Typical scenario: after long manual bookmark organization Firefox Sync adds trash from its server to my laptop or tablet.

BTW, where can I find how to override content of server's backup by current browser's settings/passwords/bookmarks? Stupid me, I set up new sync and lost this capability.