Installing FireFox for the first time

I think you're probably looking at the Firefox "Sync" feature, which exists to allow you to have all your bookmarks and history and stuff synced across multiple devices. The point of that feature is that it's only really useful if you actually have more than one device, but it shouldn't be a required step for just the normal installation - you should be able to use Firefox just as-is, without the Sync feature / creating any accounts.

To answer your second question, you'd install Firefox on another device, navigate to the Sync feature and log in using the Firefox account.

Just bear in mind that Firefox itself is just a browser. All this sync stuff and any extra features are often just additional free services that you don't really have to use if you don't want to.

gr8gonzo has a very good point.  Firefox is just a browser, but the developers have seen fit to throw everything including the kitchen sink and periodic oil changes for your car into it.  Certainly the Sync feature, which appears to be what you're running into, I have found not useful as I have no need to have the same bookmarks across multiple computers.

https://en.wikipedia.org/wiki/Firefox_Sync

I spend a fair amount of time on new Firefox installations in about:config disabling features (see list below -- and it is an old list).

Disable site push messages
  dom.webnotifications.enabled -> false
  dom.push.enabled -> false
Disable Firefox Sync
  services.sync.enabled -> false
  services.sync.engine.addons -> false
  services.sync.engine.bookmarks -> false
  services.sync.engine.history -> false
  services.sync.engine.passwords -> false
  services.sync.engine.prefs -> false
  services.sync.engine.tabs -> false
Disable Firefox Pocket
  Right-click button on toolbar, "remove from toolbar"
  browser.toolbarbuttons.introduced.pocket-button -> false
  browser.pocket.enabled -> false
  extensions.pocket.enabled -> false (52.0+)
  browser.pocket.api -> (empty string)
  browser.pocket.site -> (empty string)
Disable Firefox Reader
  reader.parse-on-load.enabled -> false
Disable Firefox referrer headers
  network.http.sendRefererHeader -> 0
Disable Firefox tile fetching
  browser.newtabpage.directory.source -> blank
  browser.newtabpage.directory.ping -> blank
  browser.newtabpage.enhanced -> false
Disable Firefox persistent cookies
  browser.sessionstore.privacy_level -> 2
Disable Firefox speculative connect
  browser.urlbar.speculativeConnect.enabled -> false
  network.http.speculative-parallel-limit -> 0 (in 52+)
Disable Firefox Hello
  loop.enabled -> false
Disable Firefox Pocket
  browser.pocket.enabled
Disable Firefox link prefetching
  network.prefetch-next -> false
Disable Firefox DNS prefetching
  network.dns.disablePrefetch -> true
Disable Firefox speculative pre-connect
  network.http.speculative-parallel-limit -> 0
Disable Firefox snippets
  browser.aboutHomeSnippets.updateUrl -> (blank)
Disable Firefox geolocation
  browser.search.geoip.url -> (blank)
Disable Firefox new features update tab
  browser.startup.homepage_override.mstone -> ignore
Disable Firefox add-ons suggestions
  extensions.getAddons.cache.enabled -> false
Disable Firefox browser opinion polls
  browser.selfsupport.url -> (blank)
Disable WebRTC peer connections
  media.peerconnection.enabled -> false
Disable Firefox video broadcasting
  browser.casting.enabled -> false
Disable plugin enumeration
  plugins.enumerable_names = <nothing>

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Since you've already installed Firefox it is, alas, too late for this advice; but due to this creeping software bloat I've found it useful to start with an older version, install that, and then come forward version by version only if there is a need for it.

Thanks, everyone. I'll try running it w/o the Sync feature and see how it goes.

OK, I installed on my mobile device, just to have it somewhere else. I haven't tried using FIrefox, on it let alone syntcing.
Thanks.

Is there a way for me to mark one of my own comments as HELPFUL? I want to give gr8gonzo primary credit, but need to add some clarifying info. Thanks.