Image didn't upload with the original question. Here it is.

You can use a custom style rule to hide that message, but I don't know how to hide it only on one site or only when it's in front of full screen content. Hmm...

Found it , with the help of reddit.com/r/firefox/ Status-4-Evar/Advanced/Detect full-screen mode.... uncheck

Hi & Happy New Year! I hate to appear stupid, but I get SO much practice, it gets easier every day! LOL :P I got all happy when I saw there was a "solution" to the overly-annoying habit that Firefox has regarding the broadcast of useless information, (at least to me, anyway), of things like: "Transferring data from...." & "waiting for..." that appear at the bottom of the window! While I'm sure this info is important to some users, there are those of us who find it very unnecessary & intrusive. In my case, I collect research for several Bald Eagle nest cams, & this involves LOTS of screenshots to document nest activity; unfortunately, many shots are ruined because of this info being captured when I take the shot, & I need to know how to make it stop happening. I read the posts above, & I clicked on the link in the "chosen solution", which is to be found on reddit's site, BUT when I clicked on the link, I got to a forum-type page, & I couldn't find anything that seemed to relate to this issue, so I copied the info below the reddit link, (above): Status-4-Evar/Advanced/Detect full-screen mode.... uncheck", as I thought maybe that would lead me to the fix, but the search came back empty. I'm sure I'm missing something here, so will you please tell me EXACTLY where I can find the solution to this problem? Thanks in advance for your help! Nuts4Mutts  :)

Assuming that you are using the Status-4-Evar Add-on then the full link should help.

https://www.reddit.com/r/firefox/comments/3n0mla/there_is_an_annoying_transferring_data_on/

Basically In the Status-4-Evar options Advanced there is a box to untick "Detect full screen mode.....etc"

Hi Kozmyk! Thanks for the clarification, but unfortunately, I don't have the "Status-4-Evar" addon, (I don't even know what it's for!), so the fix that was listed as the, "chosen solution", doesn't apply to me, so I'm back to square 1. Also, I don't know what "Twitch" is, so I've never visited the site, but it sounds like it's like YouTube...which I visit at least a few times a day. What I DO KNOW, however, is that I don't need the "transferring data from...", or "Waiting for mozilla...", or whatever info to clutter up my browser. Is there anything I can change in about:config to eliminate this unwanted info to appear at the bottom of my Firefox window? I'm a bit puzzled by its appearance, as I don't remember it being a problem before, (otherwise I would've sought help earlier...believe me!), so does anyone know if something was added/changed in one of the very recent updates, say from 40 on? If I remember correctly, there were 2 or 3 Firefox updates within just a few days of each other(?), as well as Flash popping in a couple times, too...If so, someone might be able to find what was changed in the recent updates, & maybe lead to a resolution. I'm definitely not technically inclined...well, at least not enough to know where to look at how Firefox is setup, but I AM good at following detailed instructions from those smart people who are so inclined. My "backup" browser is PaleMoon, which as you probably know is a Mozilla build, (I think that's what they call it), & it uses plugins & extensions from Firefox's add-ons site, & the way PaleMoon allows the user to decide about what's seen in the status bar is actually in the drop-down menu under, "Tools", where there is an actual entry for, "Status Bar Options", & that's where I was able to rid myself of the annoying albatross, resulting in having a window with nothing on it that I didn't put there. Unfortunately, I've spent way too much time looking for such a gold mine in Firefox, only to come up empty. That's why I was hoping that about:config or some other place in Firefox would give me the freedom to change this. Hopefully, someone will figure out when this "quirk" infiltrated Firefox so those of us who don't want this feature can do something about it. I know I'm STILL waiting for a PERMANENT way to prevent the even more annoying, "Press Esc to Exit Fullscreen"!! What a pain in the neck THAT is!! I use the little fix from someone named "Chronomaster", (I think), but I have to re-run it every time Flash is updated, (why Adobe can't figure it out is beyond me, but that's another complaint!) LOL Well, sorry for blabbing on & on...I'll get off my soap box & wish everyone a Good Night! Thanks for listening! Nuts4Mutts  :)

Nuts4Mutts said

My "backup" browser is PaleMoon, which as you probably know is a Mozilla build, (I think that's what they call it), & it uses plugins & extensions from Firefox's add-ons site,

Pale Moon is a third-party build with no Mozilla involvement. It is based on old Gecko 25 (Firefox 25) so as a result extensions for it may need to be modified due to changes or old code and may not be able to use current versions of Extensions made for recent Firefox Releases. They even host some extensions themselves for PM because of this.

Pale Moon is less significant now that there is official Win64 Firefox Releases since Firefox 42.0 as that was one of the main reasons people were even using the third-party Pale Moon builds before.