hello, could you provide your latest few submitted crash reports? please enter about:crashes into the location bar, copy the latest few report ids from there starting with bp- & paste them here into a forum reply. this will give us a better understanding what may be triggering those crashes.

bp-278c2a1f-cc10-4213-9dd4-af0912161220 20.dec..2016 07:19 bp-4da5e666-60fa-4824-9894-236452161218 18.dec..2016 12:47 bp-077bbb89-f300-40ce-8fb4-57e3f2161218 18.dec..2016 10:47 bp-1488559a-9ac1-45a9-8a6e-44c162161218 18.dec..2016 10:47 bp-b72e6b7a-02e1-42b3-971a-1898a2161218 18.dec..2016 10:47 bp-34437239-7631-4796-a5d6-f6d8a2161218 18.dec..2016 10:47 bp-8c8f6803-8ad5-48c8-b994-a9d052161218 18.dec..2016 10:47 bp-f7b35d77-994c-4426-b371-b73d42161218 18.dec..2016 10:47 bp-f81f39de-ff6f-43eb-8c42-c2d1c2161218 18.dec..2016 10:47 bp-2fe5fedd-dcd8-4ad1-a315-3e5012161218 18.dec..2016 10:47 bp-552ccbb1-1f66-4a31-a8dc-528462161218 18.dec..2016 10:47 bp-50b41479-edfd-40bd-9790-cb6702161218 18.dec..2016 10:47 bp-123cca6a-3956-4c1b-86e3-3af742161218 18.dec..2016 10:47 bp-3dc313b8-633c-49fc-890e-c1b562161218 18.dec..2016 09:39 bp-e1912bd2-d22a-4da3-b075-431cc2161218 18.dec..2016 09:27 bp-f1634e9f-bc23-42ff-8815-cc5f82161218 18.dec..2016 09:08 bp-26d03bc5-52a8-4ad4-a648-a08f82161218 18.dec..2016 08:39 bp-26a90cc1-e207-456c-8632-d06b22161218 18.dec..2016 08:26 bp-fde52729-521b-454b-9164-7a2ab2161218 18.dec..2016 08:16 bp-d3947b5f-ba4c-44a2-b072-9f7ab2161217 17.dec..2016 07:18 bp-404a1d40-5bfa-44de-b11c-3363e2161217 17.dec..2016 06:38 bp-839159ca-11f4-401d-bfd6-ce6d72161217 17.dec..2016 06:10 bp-a7973bb2-5408-44fe-af6d-db6952161217 17.dec..2016 05:45 bp-2ed19bac-0cf4-411a-bdcb-29f5a2161217 17.dec..2016 05:32 bp-0abd1d65-be97-4c3f-8dc8-2f5082161217 17.dec..2016 04:56 bp-f3ec1dad-3d08-4531-b957-2e81a2161217 17.dec..2016 04:22 bp-debdc773-abf4-425d-9b70-dbe902161216 17.dec..2016 00:18 bp-e872730f-1ca2-417a-b2f9-ae0bf2161216 17.dec..2016 00:05 bp-983e07b7-5898-4916-a8ea-b658c2161216 16.dec..2016 23:37 bp-a20afe33-6284-4098-a2af-f16472161216 16.dec..2016 23:05 bp-6beaa028-7411-4efd-8c6c-6914b2161216 16.dec..2016 23:05

thanks - does it make a difference when you try with a firefox win64bit build? https://www.mozilla.org/en-US/firefox/all/

Thank you. Where do i see which version i have already installed?

according to your useragent you have the 32bit version installed. that isn't clearly denoted in the interface yet, but it will show in upcoming versions under the firefox menu ≡ > help ? > about firefox panel.

Thank you again for the advice. I thought that Firefox homepage already sent me to the 64 bit version download page. Instead Chrome homepage suggests the 64 bit version. Weird that chrome 64 bit installs to C:\Program Files (x86) . So no more crashing uploading videos larger than 4gb with the 64 bit Firefox version.

But on my ultra book i am still using the 32 bit Firefox version and there are no crashes.
On my desktop crashes during uploading started since the 16th of December, despite i did not make any changes in the installed programs.