…or so I think. Allow me to elaborate, because I’m not active on the Mozilla forums and I feel like telling this to the greater internet. 
TL;DR - seems like if Firefox is slow to launch it might be due to the status of MS cloud services, so don’t immediately go crazy troubleshooting the issue, it might just resolve itself without any user interaction.
Yesterday Firefox would consistently take ten full seconds to open, even after clearing cache, rebooting, disabling extensions, despite not using any elaborate new tab (it’s a blank page). I had noticed the same day that in the Windows Store the list of installed apps didn’t populate even after trying for a while. After a few slow Firefox launches I checked the event viewer, and I saw errors like these.
Event viewer says..
ChkHr(GetHttpResponse( pHttpClient, hstrURL, verb, pCV, requestContent.Get(), pStatusCode, hstrResponse))
Error: Internal server error (500).
Function: _SendJsonToURLWithHttpClientForStringResponse
Source: onecoreuap\enduser\winstore\licensing\winrt\lib\iaputils.cpp (259)and
ChkHr(hrOperation)
Error: Internal server error (500).
Function: Windows::Services::Store::Internal::StoreContextServer::SendStoreRequestWithUris
Source: onecoreuap\enduser\winstore\licensing\winrt\lib\windows\storecontextserver.cpp (4365)and
SendStoreRequestWithUris( nullptr, UriHelperV7::InternalStoreEndpointExtensionUrls, UriHelperV7::InternalStoreEndpointExtensionUrlsIds::GetUserCollectionForParentProduct, cv.Get(), nullptr, nullptr, collectionsJsonString.GetAddressOf())
Error: Internal server error (500).
Function: Windows::Services::Store::Internal::StoreContextServer::GetProduct
Source: onecoreuap\enduser\winstore\licensing\winrt\lib\windows\storecontextserver.cpp (1249)and one that actually references Firefox, where “blah” was some alphanumerical identifier:
[Error] Creating UWA Work FulfillmentData = {"ProductId":"blahblahblah","WuBundleId":"blahblahblah","WuCategoryId":"blahblahblah","PackageFamilyName":"Mozilla.Firefox_blahblahblah","SkuId":"0010","Content":null,"PackageFeatures":null}
Error: The operation completed successfully.
Function: UWAInstallServiceWorkItem::UWAInstallServiceWorkItemSide note: gotta love “Error: The operation completed successfully.” Talk about a lack of self-esteem!
This morning things are back to normal, Firefox launches quickly, and this error doesn’t show. I wonder if this could have been an intrinsic issue with store API’s, or if it’s an instance where Firefox is badly coded, where the timeout for checking e.g. for available store updates should just be set to something shorter, or not interrupt the launch sequence at all.