This seems normal to me, especially if by “sleep” you mean hibernation; the network needs time to reinitialise and Firefox, in turn, must wait for the network to be available.
However, if you really want to pursue this, you know the procedure by now. Start with providing the expected system information as described (below).
Regards.
What follows is from a standard template.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with forum requirements, and the many ways to use the forum to your benefit.
Note: By virtue of using the Manjaro forum you acknowledge and agree to follow Rules and Guidelines outlined; so, you really should read them:
Required Reading
Highly Recommended
Work with us, not against us
Be prepared to provide output from commands when asked. It is equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Simply waiting for others to ask you questions can be counter-productive – typically, nobody has a 
at their disposal – instead, please help others to make informed suggestions based on information you provide.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
a) inxi command (long-form):
inxi --filter --verbosity=8
b) inxi command (short-form):
inxi -zv8
- inxi man pages (Arch Manual)
Running inxi within a chroot environment
- Add
--color=0to the long-form command, or… - Change the short-form command to
inxi -zv8c0