userChrome.css that remove or fix proton view on windows 10, with disabling megabar. It's originate from my works and some others from reddit /r/firefox. I hope this ease your pain on your computer views especially on old small laptop

userChrome.css

/*** Megabar Styler General - version 2020-07-27 ***/

  /*** General Preferences ***/
  :root {
    /* Number of pixels of enlargement when URL bar is focused */
    --mbarstyler-popout-pixels: 0px; /* [0px - 7px] */

    /* Top Bar Display or Not */
    --mbarstyler-top-bar-display: block; /* [block,none] */

    /* Font sizes (default: 13.8px for title, 10.2px for URL) */
    --mbarstyler-title-font-size: 13px; /* [13px - 18px] */
    --mbarstyler-url-font-size: 12px; /* [12px - 16px] */

    /* Rows to show without scrolling */
    --mbarstyler-max-rows-without-scrolling: 12;

    /* Bottom border for each result row */
    --mbarstyler-bottom-border-width: 0px; /* [0px or 1px] */
    
    /* Match display style */
    --mbarstyler-match-weight: 700; /* [400,700] */
    --mbarstyler-match-background-opacity: 0.0; /* [0.0,0.05,0.1] */
  }

  /*** URL bar enlargement or lack thereof ***/

  /* Compute new position, width, and padding */
  #urlbar[breakout][breakout-extend] {
    top: calc(5px - var(--mbarstyler-popout-pixels)) !important;
    left: calc(0px - var(--mbarstyler-popout-pixels)) !important;
    width: calc(100% + (2 * var(--mbarstyler-popout-pixels))) !important;
    padding: var(--mbarstyler-popout-pixels) !important;
  }
  [uidensity="compact"] #urlbar[breakout][breakout-extend] {
    top: calc(3px - var(--mbarstyler-popout-pixels)) !important;
  }
  [uidensity="touch"] #urlbar[breakout][breakout-extend] {
    top: calc(4px - var(--mbarstyler-popout-pixels)) !important;
  }

  /* Prevent shift of URL bar contents */
  #urlbar[breakout][breakout-extend] > #urlbar-input-container {
    height: var(--urlbar-height) !important;
    padding: 0 !important;
  }

  /* Do not animate */
  #urlbar[breakout][breakout-extend] > #urlbar-background {
    animation: none !important;;
  }

  /* Remove shadows */
  #urlbar[breakout][breakout-extend] > #urlbar-background {
    box-shadow: none !important;
  }

  /*** Top "Blue Bar" Display ***/

  .urlbarView-row:first-of-type {
    display: var(--mbarstyler-top-bar-display) !important;
  }

  /*** Font Sizes and Scrolling ***/

  /* Title font-size */
  .urlbarView-row .urlbarView-title {
    font-size: var(--mbarstyler-title-font-size) !important;
  }

  /* URL / action font-size */
  .urlbarView-row .urlbarView-secondary,
  .urlbarView-row .urlbarView-url,
  .urlbarView-row .urlbarView-action {
    font-size: var(--mbarstyler-url-font-size) !important;
  }

  /* Set max-height for items visible without scrolling */
  #urlbarView-results, #urlbar-results {
    height: unset !important;
    max-height: calc(2.5 * (var(--mbarstyler-title-font-size) + var(--mbarstyler-bottom-border-width)) * var(--mbarstyler-max-rows-without-scrolling)) !important;
  }
  #urlbar-results {
    overflow-y: auto !important; 
  }

  /* Clean up extra spacing at the top and bottom */  
  #urlbar-results {
    padding-top: 0 !important;
    padding-bottom: 0 !important;
  }

  /* Subtle border between results */
  .urlbarView-row:not(:last-of-type) {
    border-bottom: var(--mbarstyler-bottom-border-width) solid rgba(0, 0, 0, 0.1) !important;
  }

  /* Match Styling Like Fx43-47 */
  .urlbarView-row:not([selected]) .urlbarView-title strong,
  .urlbarView-row:not([selected]) .urlbarView-url strong {
    font-weight: var(--mbarstyler-match-weight) !important;
    border-radius: 2px;
    box-shadow: inset 0 0 1px 1px rgba(0, 0, 0, calc(var(--mbarstyler-match-background-opacity) * 2));
    background-color: rgba(0, 0, 0, var(--mbarstyler-match-background-opacity));
  }
  [lwt-popup-brighttext] .urlbarView-row:not([selected]) .urlbarView-title strong,
  [lwt-popup-brighttext] .urlbarView-row:not([selected]) .urlbarView-url strong {
    box-shadow: inset 0 0 1px 1px rgba(255, 255, 255, calc(var(--mbarstyler-match-background-opacity) * 2));
    background-color: rgba(255, 255, 255, var(--mbarstyler-match-background-opacity));
  }
  [lwthemetextcolor="bright"] .urlbarView-row:not([selected]) .urlbarView-title strong,
  [lwthemetextcolor="bright"] .urlbarView-row:not([selected]) .urlbarView-url strong {
    box-shadow: inset 0 0 1px 1px rgba(192, 192, 192, calc(var(--mbarstyler-match-background-opacity) * 4));
    background-color: rgba(192, 192, 192, calc(var(--mbarstyler-match-background-opacity) * 3));
  }

/*** End of: Megabar Styler General ***/


/*** Megabar Styler One-Offs - version 2020-05-31 ***/

  /*** One-Off Search Button Preferences ***/
  :root {
    /* One-Offs Display or Not */
    --mbarstyler-oneoffs-display: flex; /* [flex,none] */
  }

  /* Show or Hide the One-Offs Search Icon Bar */
  #urlbar .search-one-offs:not([hidden]) {
    display: var(--mbarstyler-oneoffs-display) !important;
  }
  /* Shorten the One-Offs Search Icon Bar Vertically */
  #urlbar .search-one-offs:not([hidden]) {
    padding-block: unset !important;
  }
  /* We don't need the text */
  #urlbar .search-one-offs .search-panel-header {
    display: none !important;
  }

/*** End of: Megabar Styler One-Offs ***/

/** Disable GUI Shadows */
* {
  box-shadow: none !important;
}

/* Disable Hamburger Menu */
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

/* #PanelUI-menu-button { margin-right: 140px !important;} */
/* End of Disable Hamburger Menu */
.tab-close-button:not([pinned]) {
    display: -moz-box !important;
}

/* make firefox great again */
/* #main-window[uidensity="compact"]  {
  --tab-min-height: 28px !important;
}

#main-window[uidensity="compact"] .tab-close-button {
  width: 24px !important;
  height: 20px !important;
  padding: 2px 5px !important;
} */

.tab-stack {
  margin-top: 0px;
}
.tab-background {
  margin: 0px !important;
  border-bottom-left-radius: 0 !important;
  border-bottom-right-radius: 0 !important;
} /**
.tab-background[selected="true"] {  
  background: linear-gradient(to bottom, rgb(64, 70, 168) 2px, var(--toolbar-bgcolor) 2px, var(--toolbar-bgcolor) 100%) !important;
}
*/
.tab-context-line {
  margin-top: 26px !important;
  margin-left: 6px !important;
  margin-right: 6px !important;
}
menupopup > menuitem, menupopup > menu {
    padding-block: 0.15em !important;
}
.panel-subview-body > * {
	padding: 2px 6px !important;
}

.editBookmarkPanelBottomButton {
  min-width: 135px !important;
  width: 50% !important;
}
/* Tab List Button */
.panel-subview-body > * {
	padding: 1px !important;
}

.subviewbutton {
  padding: 0px !important;
}